The power supply should be ATX 20pin connection and also a 4pin connector, look at this picture below so you know you have the power connections in the right places>>
On the top left of the picture, the big cream 20pin socket is where the 20 pin connector goes.
In the middle to the left you will see another cream 4pin socket is where the 4pin connector goes.
If your power supply does not have the 4pin connector the pc won't boot up, you will have to go out and buy one.
If you have all the connections right and the on/off button is connected to the front panel properly, take out the system memory, clean it with a clean, dry, soft pain brush and also clean the slots too, re-seat the memory and reboot the machine to see if it boots up.
Good luck and thanks for using Fixya!!!