APFS decryption stuck
Solution 1:
Erase the volume. This can be done in Recovery Mode if necessary using the Terminal, which can be opened using the menu. Restore from a backup, then enable FileVault2, leaving the computer on and plugged in to allow the encryption to complete uninterrupted.
This will erase all of the data on the volume you specify and leave behind an empty unencrypted volume:
diskutil apfs eraseVolume <apfsVolumeDisk> -name <newName>
Example:
diskutil apfs /dev/disk2s2 -name "My HDD"