Broken Power Switch
Hello, will it turn off if the switch is held down for longer ( can take up to 5 secs. ).
If it doesn't the switch may have been accidentally disconnected from the wiring.
If you're confident enough to fix it yourself, make sure the plug is disconnected from the mains along with any other connections to your PC.
Take the left hand side panel off of the tower, there should be some screws on the back holding it in place. Put on an anti-static wrist band and check on the board for the small connections that lead from the area of the power switches on the front of the tower.
The connectors onto the board are usually small flat black plastic connectors and are usually labelled, ideally you should refer to the motherboard manual which will have a diagram of the board and which connectors are which.
If all the connectors are in the right place, you will need to check the connections on the switch itself. Carefully remove the plastic panel from the front of the tower, there will be some small tabs holding it in place, be careful not to break them or you may have problems putting it back on.
Once you have removed the front panel you should be able to check the wiring that is connected to the switch. If any of the connectors have come loose, simply slot them back on, if they need soldering then leave it to somebody who can repair the switch for you.