The issue was resolved this morning, and the server is running normally. I'll post the solution to help others in the future. I cut power to the rack overnight. This morning I removed all the power cables from the server and the UPSs. I swapped PSUs 3 and 4, and re-seated PSUs 1 and 2. I again drained power from the board by holding the power button down for 30 seconds. I then plugged everything back in, and checked all the cables. I restored power to the rack and powered on the server and it came on as normal. So I guess it was nothing special, just swapping PSU positions, re-seating them, checking and re-seating cables, and draining the board. Didn't work the first few times but going through a couple times didn't hurt and everything seems to be running fine now.
Our PowerEdge r920 server was powered off unexpectedly (along with the entire rack), and since then we have not been able to power it back on. The following two errors are showing up on the LCD display:  VLT0222: The NDC NDC PG voltage is outside of range. Contact support. VLT0204: System board fail-safe voltage is outside of range. Contact support.  So far I have done the following:  The cause of the power outage was the UPSs on the rack dropping all power output to the rack, so everything on the rack lost power. Since then we have gotten power supplied back to the rack, and everything but the r920 server came up normally. So I'm not sure if such an abrupt loss of power, or possibly a power surge could be the cause of the voltage errors, or maybe something else. Any and all help is appreciated. Thanks. 
PowerEdge r920 voltage outside of range