Is your PC overheating?
Check the CPU and mainboard temp.
Is the PSU faulty?
Has windows a error in error log for the time it powered down?
Is it a memory fault?
It could be any, some people on here will be able to suggest ontesting these, but I use software which is not free, sorry.
PS. You could try SiSoft sandra, do a search on Google, for the free version.