Grrrrrrrrrrrrr. Do you have a maintenance agreement? If you do call them back and tell them to get there asap.
Sounds very much like the firmware is corrupted. If you cant get to diagnostics, here is what you can try.
On the back of the machine open the panel to access the memory, remove the memory and power the machine up with the memory removed. Let it fault out, then replace the memory and start it again. See if that works.
Then remove and reseat the main board by turning the orangish knobs and sliding the board out and back in.
It really sounds like the upload corrupted and Xerox needs to resolve this for you.
They will try and say it's not their fault, but if they directed you to do the upgrade, they are at fault and should fix it for you.
Unless, and I very much doubt the old software is on their site, no you cant go back to the old firmware, you may try reloading it and see if it loads correctly.