Hi [NAME],   Let's bring the server to minimum to POST components, remove all components except the list below, then try accessing to the BIOS.  If it works, one of the components which was removed is not working on the server. Install the components back one by one to check the issue. If it doesn't work, if iDRAC is accessible, try reseting the iDRAC by referring to this link, preferrable a hard reset: [URL]
I have a T420 on Xerox BIOS 1.5.1 I was planning to replace the CPU's , and in doing so I needed to Update the BIOS, I have used a USB and booted into dos and installed the latest BIOS 2.9.0 then realized I should have stepped up the updates. Rolled back the BIOS  thinking I would be back to 1.5.1 but I am now on 2.0.22. I cannot boot from USB device, enter bios menu or boot setup. IDRAC still connects and is on version 1.51.51 (Build 01) I cannot update IDRAC as when I try to upload any files I get below error message. "OSD35: Lifecycle Controller is not enabled. To enable Lifecycle Controller, reboot the server. During POST, press F2 to enter System Setup. Go to IDRAC Settings -> Lifecycle Controller, select enable, and save the changes. " I cannot enter bios menu or boot setup  
T420 Lifecycle controller disabled