Hi,   You can refer to our support page for T420. You would need to search for the RAID controller's model and download the firmware.  [URL]   I would recommend to update the firmware via Lifecycle controller. Are you able to access to it?  
Power outage for several hours today and after power was restored server would not boot. At boot get the following error. RAID Adapter Firmware Failed Validation!! Adapter needs to be reflashed. Connected to LifeCycle and ran diags. Hard Drive errors with no drives detected. Can the RAID firmware be flashed from a bootable USB flash drive? Can someone point me to a link to download?
PowerEdge T420 boot error