There are several possible causes for this error. To fix it, you must determine which component is defective, ranging from the cables that attach your monitor to your PC to the monitor itself or even your PC's video card.
1. Turn off your computer.
2. Unplug the cable running from your monitor to your PC and plug it back in, making sure the connection is firm. The most common cause of this error is a loose cable.
3. Reattach the cable running from your monitor to your PC. If the monitor is still not displaying a signal after firmly reattaching the cable, the problem could lie with the cable, especially if the cable has visible damage.
4. Replace your monitor with another monitor if possible. If the "No Input Signal" error still appears, the problem does not rest with the cables or the monitor, but with your PC.
5. Open your PC case and locate your video card. Remove the card and then replace it firmly in its slot or, if possible, insert the video card into another slot on your motherboard. A video card that is not making a firm connection will not display images to the monitor. Close your PC case and test the monitor again.
6. Replace your video card completely. Your video card may suffer from internal damage that is not allowing it to output a signal.
NOTE:: If you are still not receiving a signal even after replacing your video card, you may also need to replace the computer's power supply and, in a worst-case scenario, your computer's motherboard.
If your computer is still under warranty, arrange for an official repair before you open up your computer. Often, opening your computer yourself will void your warranty.