It may be that this is one of those "trying" occasions which an O/S does to test your patience!
It could be that the HD is failing, but if you have saved anything vital to a safe place, then it is surely time to do scandisk with a thorough surface scan. If you do this it is worthwhile arranging to do something else while the amchine gets on with the job, ( I couldn't stand the tension waiting to see......)
If all is well, it will eventually finish, declaring no bad sectors and then behave itself.
If it does show bad sectors, this is the beginning of the end and I would replace it - soonest.