Blue (Prusa) Screen of Death  

Blue (Prusa) Screen of Death

I've been trying to communicate between my Prusa i3 MK3 and an external Arduino. I need this Arduino to do the controlling of a couple of additional pieces of hardware. I'm trying to do the communication through the J19 connector, much like a Raspberry Pi can communicate with the Prusa.

This communication HAS been working in the past, but I just haven't used it in a while, concentrating on other developments. But today I had a problem with the communication. I tried all sorts of approaches, and at one point, having restarted the printer, it just never gets past the first display on the screen, doing its initialization I presume. Instead it just shows the initial "Original Prusa i3 : Prusa Research" screen. Both fans on the print head are running, but nothing else happens. It just stays at that blue screen.

My project involves both writing software for the Arduino as well as making modifications to the Prusa Firmware. I've been doing this successfully for several months. I'm a career software engineer, so have taken a cautious approach to all this. I suppose it's POSSIBLE that something interrupted a recent flashing of the firmware, which I always do via the flash option in the Prusa Slicer, but I don't think so.

It isn't possible to go back and flash the original firmware, since I can't get past that first blue screen.

There's supposed to be a Factory Reset that you can do, but pressing the specified buttons to do that does nothing. Unplugging/replugging does nothing. Hitting Reset does nothing.

Is there some way to get past this, to get it working again?