Power cables often tend to loose from the power unit of computer over time. This may be due to pulling the power cables or disturbing it. It may also be result after your computer is moved from one location to another.
The image shows the wire coming from the power supply unit. Make sure that the wire coming from the power supply unit is firmly inserted.
Steps to fix:
1. Switch OFF your computer. Remove the connection from your wall socket.
2. Check the power cable behind the computer case.
3. Remove the power cable, gently blow to remove any accumulated dust. Fix the cable back to computer power unit firmly.
4. Check the connections in the UPS (if you have any), to ensure that the connections are firm.
5. Check whether the computer power cable is securely plugged in to the wall socket.
Try to switch ON your computer after doing these steps. If your computer powers ON, you can rest easy that you have successfully troubleshooted your Computer.