My PC turns on but nothing is happening!?

My PC, i press the On button and it sounds like its on, the fan is going, and the screen is on but its not connected, so the screen has no signal, the blue flashy light that usually comes on isn't on either, its just gone... lately it had been blue screening and shutting off(not shutting down) a lot, but it seemed to work now and then, it's really expensive to fix, i tried plugging stuff in and out and all, Does anyone know what the problem could be? if im not being clear enough just ask a Q and il try add more detail!