Computer powers on but no output to display, and smetimes boots

Hello,I have an Hp pro 3500, its been a month since it had a problem of turning on and not displaying anything to screen, but sometimes after a few power ups it turns on, I've lived with that and it seems now the problem has escalated, its similar to recently discussed questions, but not entirely, it turns on, blue light on, fans on, but no output to display, I thought the problem was the battery because whenever I removed and put it back it would sometimes turn on, so I baught a 3 new different battries just to be sure, the proble still persists, worse now cause it has stopped turning on. Please help me, is my motherboard dead? Or my cpu? Because whenever it manages to boot it works just fine.