We had a power outage this morning, thanks, PG&E, and the Raspberry Pi that runs my IoT hub wouldn’t reboot.
The SD cards in a PI can get corrupted in case of a power cutoff, so checking for that was the next, best, right thing.
I put the SD card in my laptop running Ubuntu:
Ubuntu makes an alias for a SD card at /media/$USER/.
You can there in the File viewer and open the file system
in Disks to find which dev it's mounted as.
sudo fsck.fat -V /dev/mmcblk0p1
[sudo] password for $USER:
fsck.fat 4.1 (2017-01-24)
0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
1) Remove dirty bit
2) No action
Starting check/repair pass.
Starting verification pass.
Perform changes ? (y/n) y
/dev/mmcblk0p1: 216 files, 79946/516191 clusters
fsck.fat again to confirm the repair, then unmount the SD and try booting the PI again.