no signal going out fromtower system?

when i pugged in my Intel Pentium 4 Desktop to 3.2Ghz it said (enter sleeping mode). I'v asked about and tested my monitor and other parts and found that it's not giving a signal out at all.
not sore if its the motherboard or the procesor.what would you surgest i do as i can fit most of the parts my self.please could you help as i'm on a budget on what i spend before i decide if its worth doing.i payed £220 for it.
i wpuld be gratefull for you help.
kind regards
anthony