In shutdown before fast startup, Windows stores the OS state on the drive, similar to hibernation (though without running application state) in hiberfil.sys. If that file were to be corrupted, it could prevent a subsequent fast boot.
Try temporarily disabling hibernation, either from the power options menu, or by opening an elevated-rights CMD dialog and entering:
powercfg.exe /hibernate off
This should delete hiberfil.sys, or allow it to be manually deleted.
Then re-enable hibernation with
powercfg.exe /hibernate on
Hopefully, that will fix the issue.