Chances are it may be your power supply.
Your best bet is (firstly) to disconnect any additional peripheral devices than may be hogging power (ie. CD/DVD USB devices) and just have your CPU, MB, HDD, mouse and keyboard connected and try to power up. If this does not work, your next step is to try an alternate PSU (if you have a spare PC, or a friends PC).
Alternatively you can take it to a PC shop (not Harvey Normans but an actual PC seller) and see if they will let you try one of their PSUs (usually they will have one or two set up anyway for fault testing PCs). Then you will be able to find out if it is your power supply, and while youre there, you can buy a new one (if needed).
Otherwise (worst case) it may be your motherboard...
Hope that helps