I mistakenly wrote an x64 Ubuntu iso to my SD card.
When I tried to boot my Orin Nano off it, I got a UEFI shell prompt. That was not something I’d seen before, which confused me, and googling didn’t really come up with any good suggestions.
Only when I went to rewrite the SD card a second time did I notice I picked the wrong image file the first time and, of course, picking the right image worked fine.
So, I leave this here so the next person who makes a similar mistake has something more helpful in their search results.