hi a couple of days ago my pc started crashing after being turned on for awhile. when it crashes everything is frozen and i cant do anything but turn it off via power button, any ideas at what the fault is please?

This sounds like it may be a heat issue.

Power down and remover the side of the case and see if there has been a build up of dust and debris, especially around the cpu heatsink and fan.

If there has then obtain a can of compressed air and give it a good blow out.

I'll have a look then, cheers.