One of the windows files was corrupt, you should have left it to complete the repair when Start-up repair is checking your programme for problems.
switching off during the repair would have cause more problems hence the amount of time rquire for it finally to fix itself.
how dangerous is it to the well-being of the operating system?
its repaired itself so no more danger.
You need to have a backup routine
Make sure all your important data is backed up to and external source or two if a hard drive fails then noway it can repair itself.
regular clones of the drive could be a life saver.
Treat this first failure as a warning and do not be caught out
However it may never fail again but better safe than sorry.