Firstly: Don't unplug your PC, you could cause irreparable damage.
Use the Reset button.
This happens to my old plodder (8yo) every now and again, but not that often. By pure chance it happened this morning. My guess is that, for some reason, the Hard Drive did not start, but to be honest I have no idea.
What I have done: I have no real intention of buying a replacement but I have started saving.
I have set in hand a Backup System. I upgraded it from XP to W7 18 months ago and that has a Backup Facility built in. I bought a WD 'My Passport' and backup every Sunday Night. I also installed Nero 'Backitup' which I use to backup my .pst File and Accounts, every day.
With luck, if everything goes tits up, I shall be able to resume normal service, within about 10 days, by using a Mates PC to order a replacement, on-Line.