Remove the HDD from the system and give it a rest, the more you'll try to access it the more damaged it will get.
Purchase a new HDD and reinstall OS.
Purchase an external HDD dock or enclosure and insert the old HDD in it. Try to access the old HDD on your PC (or another system) and retrieve the files.
Do I really have to unplug it? Or can I just not switch on the PC or even supply power to it?
Thank you for the solution. Just a couple more queries, you're saying I should make this into a temporary external HDD with a dock and plug it in? Won't it just whirr again in the dock? And can I recover everything? (Provided it doesn't go crazy?) Even my game installs?