Now that you've excluded the power supply from the list of possible causes two things come to mind that you can give a try:
1) You mentioned you reseated the "main" ribbon cable. The HX Effects actually contains two primary ribbon cables:
The wide main ribbon cable connecting the main processor board to the display/footswitch board.
A smaller, secondary ribbon cable that specifically routes power and control data to the upper scribble strip/LCD assembly.
If the main processor tries to boot, queries the secondary display board, and receives a short circuit or no response due to a bad connection, the bootloader will crash and force a reset loop.
Open it back up. Disconnect both ribbon cables. Clean the gold contact fingers on the ends of the cables using 99% Isopropyl Alcohol and a Q-tip. Ensure they are seated absolutely straight and deeply into their Molex plastic collars before locking them down.
2) Because you just did an update while the unit was in a semi-broken state, it is possible there is a corrupted global settings file or "problem preset" causing a boot crash.
Even with blank screens, you can force a hardware reset using the physical switches:
Turn the unit OFF.
Press and hold Footswitch 6 (top right switch) and the TAP switch simultaneously.
Turn the unit ON while holding those down. Keep holding them for about 10-15 seconds.
If it successfully clears the state, the boot loop might stop, allowing the displays to spring back to life.
Good luck finding the cause of the issue, share if you find it.