My always-on Win8.1 back-up computer started sulking the other day, slow to open programs or browsers etc. So I clicked 'restart'...
...the 'restarting' symbol stayed there for ages, then a black screen, then finally it restarted and has been much better than it used to be ever since
But the whole restart sequence must have taken 5-10 minutes in total.
The ways of Win8.1 are weird and less-than-wonderful.
Try just booting it and leaving it for a while to sort itself out.