Could you give me some more details about the issue? What happens when you press the power button? Do you hear some unusual sounds? Are the cooling fans spinning and could you tell me what is the beeping like? Is it like three beeps in a row? Is there a signal on the monitor or all remains black?
Please disconnect all but the power cables of motherboard and one stick of the RAM memory. If the CPU feel hot ant the cooling fan does not start, please confirm that there is enough thermal paste applied. Is this a new PC or you have been using it for some time?
Hope this helps