Try to disconnect the power cord form the computer, wait a few seconds. Put it back in. Wait a few more seconds. Now, try to power on the computer.
If that did not solve it, then you must disconnect the power cord again. Open your computer and look for the ATX port.. It is usually located by the long RAM sticks. You will see connectors,( 24 ports), that will be going into the motherboard. Unplug the connector bundle. Reconnect it. Now close your case, put the plug back into the computer and try to boot it up.