How old is your PC? Did you build it? If so, what are the parts? If not, what brand and model is it?
Reseat your RAM and graphics card and see if that makes a difference. Does the CPU fan come on when you turn it on?
If it is more than a few years old, it may need a new CMOS battery. This is typically (but not always) a CR2032 lithium coin cell on the motherboard. They are readily available in discount stores (e.g. Wal-Mart), do-it-yourself stores (e.g. Home Depot) and office supply stores (e.g. OfficeMax).