I doubt you need to downgrade firmware, although that might help temporarily. It is likely that the database got corrupted. Load a known valid database image on a the root of a memory card and name it image.ci3
In Crimson software, from the file menu choose File > Save Image, and then it will create a complete image, firmware and all. That is the surest way to recover from an endless reboot cycle. The alternative is to catch the unit when first powering up and click the send button in Crimson (use a USB connection). If your timing is slightly off, try again (power cycle, click send just after the logo screen appears).
Using a valid image.ci3 file on a memory card eliminates the need for perfectly timing the download attempt.
I have done this apx. 3 times in ten years and was always able to recover with one of these methods. One time I had an uh-oh in a program I was calling on startup. One time I had a corrupted database that I fixed by starting a 2nd instance of Crimson and dragging the important and known good parts from the corrupted file into.
I forget the 3rd occurrence, but was something similar.