It happens on rare occasions to the best of us :)
Probably something that hadn't finished 'doing it's thing' when pc was shut down and Windows didn't wait for it to finish (it's supposed to wait)
If it happens on a frequent basis then needs investigating as could also be hardware related.
Usually closing down pc and starting up again clears the problem. Last time it happened to me was about 4 months ago. Never did find out what caused it but it hasn't happened since.