If you press the power button on your computer tower and nothing happens, it is most often caused by a damaged or destroyed power supply. Sometimes it's a simple fix though, and there's a few things you can try to get the machine back up and going.
-Check the power cable. Make sure it is securely plugged into the back of the computer. Follow the cable back to the outlet and make sure it is plugged in there as well.
-From the outlet, plug another device in to make sure there is power being supplied from the plug.