Sorry to hear about all the troubles. If it can be started after a further reset, I would disable the Sleep mode in bios entirely, and start from there. Anyway either there are a lot of people with your issue that are not posting here, or actually the motherboard is worth being changed (personal opinion). This seems , by your words, a glitch that started after a certain time, and from that point keeps repeating. It could be a slight defect in a component of the motherboard, could be a bios update or any low level update triggering a random malfunction into a component that maybe by bad luck is not 100% as should be as specs, etc ( could be the psu not recovering from an induced state ). Sadly you are correct about the onsite support... the volumes of these systems (sales) are not high and the subcontracting companies tend to spend time "teaching" their techs about the most common to be repaired units ( when I had need for a fix, I directed the tech in what to disassemble and the order.. since I studied the hw manual. For him it was the first time seeing a 7820 and I'm 200% sure he wasn't shown a manual for it )
I bought my dell tower with the top build parts last year(2021).   Today, it suddenly can not start as usual.   I followed the online technical service to reset the power by unplugging the power and pressing the power button for the 20s.   It works the first time, then I keep working with my workstation for a day.   Then suddenly it can not start again, this time getting worse. I can not start the workstation by resetting the power.   And Dell promises to send a technician to help. for your information, I have waited from Thursday 3rd Nov until 8st Nov 2021 to get a technician to help.   Dell sends the motherboard to the technician and asks them to replace it, however, workstation 5820 is not the regular type of Dell product, and the technicians are not trained to disassemble the machine.    So the technician decided to help me test data rather than tearing the machine apart, which might cause extra damage.   The reason that technician doesn't suggest I replace the motherboard was due to two reasons. First, Dell didn't send the motherboard properly, and the part might have already been damaged from transition(the technician showed me the part of the motherboard already split), Second, the technician didn't train to replace this particular type of device and he told me most Dell technicians didn't train to fix this complex type of device, so there was no technician who would answer this case, which keep me waiting longer than usual.   I asked him what if the problems happen again, and what would he suggest I do with it. He told me the better way is to send the workstation back to Dell and let a professional run the test for it.   After the data test, everything seems ok and the technician helps me to reset the power and sleep mode from hybrid to regular mode, which he assumes might cause the abnormal start problem.   After he has gone, the workstation works normally for days.   Today the problem shows again, I went to Dell's website and the online assistant told me they will send technician to help.   Can you all see the problems of this? 1. Assistants can not figure out what might be the problem. 2. Online assistant decided to send a technician who does not know how to deal with this kind of device. 3. Online service told me I can not send the device back to Dell(they only accept laptops for this), which is what their onsite technician suggest I ask Dell for a box to send it back if the problem happened again. 4. Technicians don't know how to disassemble the device to replace the motherboard might cause damage to the workstation, also the motherboard is already damaged from the transition. Who should be responsible if the data from the workstation damaged by this unthoughtful action?   I suggested all my friends and family pick Dell as their first choice before. I've changed my mind at this moment.   I spent more than $5000 to expect a better sales service, not like this.   Here is the quote from my 5820 towers. Quantity Subtotal Precision 5820 TowerEstimated delivery if purchased today:Mar. 15, 2021 $4,687.34 1 $4,687.34 Description SKU Unit Price Quantity SubtotalPrecision 5820 Tower XCTO Base 210-ANJK - 1 -Intel(R) Core9(TM) i9-10940X 3.3GHz,(4.8GHz Turbo,14C, 19.25MBCache, HT, (165W) DDR4-2933 Non-ECC) 338-BUNP -1 -Intel(R) Core(TM) i9 Label 389-CGEV - 1 -CPU Heatsink 5820 Tower 412-AALF - 1 -Windows 10 Pro, 64bit English 619-ANUF - 1 -Intel NVMe PCIe SSD (Front PCIe FlexBay) 414-BBBV - 1 -Intel Integrated controller (RST-e) with 1-2 Front FlexBay NVMe PCIeDrives 321-BDWI -1 -Dell M.2 carrier 400-AVDR - 1 -M.2 1TB PCIe NVMe Class 40 Solid State Drive 401-ABJR - 1 -Dell M.2 carrier 400-AVDR - 1 -M.2 1TB PCIe NVMe Class 40 Solid State Drive 401-ABJR - 1 -No Hard Drive 400-AKZR - 1 -No Hard Drive 400-AKZR - 1 -No Hard Drive 400-AKZR - 1 -No Hard Drive 400-AKZR - 1 -No Optical 429-ABER - 1 -Slim filler panel (no opt.) 429-ABEP - 1 -No RAID 780-BBCJ - 1 -Dell Limited Hardware Warranty Plus Service 997-7163 - 1 -Onsite/In-Home Service After Remote Diagnosis, 3 Years 997-7165 - 1 -Nvidia Quadro RTX5000, 16GB, 4DP, VirtualLink (XX20T) 490-BFFI - 1 -Adapter 6+2 connector to 2x 6+2 connectors RTX chassis 492-BCQN - 1 -Precision 5820 Tower Core X 950W PCIe FlexBay Chassis CL 321-BFGM - 1 -T5820 950W Regulatory Label (DAO) 389-CGKH - 1 -Placemat 5820 Tower MUI DAO 340-BYNM - 1 -64GB, 4x16GB, DDR4 UDIMM non-ECC memory 370-AFGJ - 1 -US Power Cord 470-AAKG - 1 -Thank You for Choosing Dell 340-ADBJ - 1 -No Additional Network Card Selected (Integrated NIC included) 555-BBJO - 1 -Page 2 Dell Marketing LP. U.S. only. Dell Marketing LP. is located at One Dell Way, Mail Stop 8129, Round Rock, TX 78682 SERI Guide (ENG/FR/Multi) 340-AGIK - 1 -No Stand included 575-BBCH - 1 -GPT is 100% required for all order 411-XXYB - 1 -No Accessories 461-AABV - 1 -No Microsoft Office License Included – 30 day Trial Offer Only 658-BCSB - 1 -OS-Windows Media Not Included 620-AALW - 1 -Trusted Platform Module (Discrete TPM Enabled) 329-BBJL - 1 -Dell Optimizer for Precision 640-BBSC - 1 -Resource DVD not Included 430-XXYU - 1 -No Out-of-Band Systems Management 631-ABML - 1 -Ship Material Tower 5820,7820 328-BCRU - 1 -SHIP,PWS,LNK,NO,NO,AMF 340-AEYP - 1 -Performance Optimized 370-AAIP - 1 -BIOS match checked back to factory 444-BBBG - 1 -BIOS binary check enabled and verified 444-BBBS - 1 -SupportAssist 525-BBCL - 1 -Dell(TM) Digital Delivery Cirrus Client 640-BBLW - 1 -Enable Low Power Mode 658-BBMQ - 1 -Dell Developed Recovery Environment 658-BCUV - 1 -Not ENERGY STAR Qualified 387-BBBE - 1 -No Additional Keyboard 580-ABIS - 1 -No Anti-Virus Software 650-AAAM - 1 -Firmware and Driver for Intel 8265 Wi-Fi + Bluetooth module 555-BEVY - 1 -Intel® Dual Band Wireless AC 8265 (802.11ac) 2x2 + Bluetooth module 555-BEVZ - 1 -Intel Dual Band Wireless AC 8265 (802.11ac) 2x2 + Bluetooth module 555-BEWB - 1 -No UPC Label 389-BCGW - 1 - Subtotal:Shipping:Estimated Tax:Total: $4,687.34$0.00$398.46$5,085.80   Maybe someone from the community can help me to find out what might cause the starting problem. Also, I would like to hear any suggestions from you about how should communicate with Dell. I really want my workstation to get fixed but I don't want Dell to keep doing the wrong decision for me, please tell me the best way to do it if you have any thoughts.      
Dell Percision 5820 tower can not start