I had an odd issue happen with my 5" Nextion enhanced display last night and I thought I’d share the resolution here in case others have the same problem (or I forget and go searching for the answer again ).
I find that using the SD card to update the display firmware is extremely useful, especially during the development cycle when I’m getting the MCU and the Nextion code working together. I probably did 20 or 30 updates yesterday. At the end of the day, the Nextion refused to recognize the SD card and kept asking if I had formatted it FAT32. Of course I had, since I had been using the same card all day long.
With my preferred update method now apparently gone, I connected up a 3.3V USB/TTL adapter to the display and uploaded the new program. It uploaded with no problem. Just for kicks, I tried the SD card method again, and NOW IT WORKS!
Apparently the last time I uploaded the program from the SD card I shut the power off too quickly and left the display in some odd state. Going through the serial upload and reboot process appears to have fixed the problem.