Sounds like a cable may be pressing against an installed component and unseating it.  Re-seat cards, RAM modules, heat sink, CPU fan, chassis fan, etc. Make sure all cables are fully/firmly connected to motherboard, fans, drives etc, and not hitting any component when the case is being closed.  As for not booting from USB, look in BIOS setup to see if there's an option to allow USB booting and enable it. Then try booting from USB again, just to make sure that will work.
Hi, I have a Dell inspiron PC with 8GB ram and W10. The machine has been bullet proof. Recently, I decided to upgrade my RAM from 8 to 16 GB. However, when I tried to boot up I experienced the following message; (Black Screen) START PXE OVER 1PV4 (and then) START PXE OVER IPV6 (and then) Auto Repair (doesn't repair!). I removed the new memory but the problem continues.  It is sometimes temporarily resolved by switching off the machine and reseating the original memory card from one slot to the other. Searching on line for a solution, it has been suggested that changes have been made in the Boot Menu that need reconfiguring or that I should disable IPV4 and IPV6 and that will solve the problem. I’ve tried both of these options and nothing has worked. Boot Menu order is; SAMSUNG, Ipv4, Ipv6, Windows Boot Manager. I don’t know what has happened but I’d certainly appreciate a way to resolve it. Thanks Steve  
START PXE OVER IPV4 - Help!