I had a malfunctioning M3 that was hanging at the boot load screen, and this is what I did to fix. I probed around looking at power on the interfaces and noticed I was missing 5V on the USB controller (ISP1761). In the schematics for the M3, I noticed the chip sourced its voltage from the KLM-2700 card, and the card seemed ok when I measured it previously. This took me a hot minute to find. Pulling the board out and inspecting the card showed a fuse on the bottom side of the board that had went. After replacing the fuse, the M3 booted up instantly.
KLM-2700 (top side)
KLM-2700 (bottom side), note fuse (FU2)
FU2 buddy! Probably the best reference designator ever, and appropriate.
Just add replacement fuse across the lands and you should be good to go. If your boot problem was due to the same issue of course.