When the "no-input signal" error message appears on your monitor, take a few factors into account, each with varying levels of computer skill required for the resolution. As with all computer problems, turn off both the monitor and the computer for 60 seconds, then restart the system before beginning in-depth troubleshooting.
Power-Save or Hibernation
When a computer is in power-save, or hibernation mode, it stops sending a signal to the monitor. Resume functionality by pressing a key on your keyboard or by pushing the power button, or by moving the mouse.
Bad Monitor Cable
After you ensure that the computer is sending video to the monitor, verify that the monitor cable is secure and tight at both ends of the cable. If it's secure, remove the cable and verify that there are no bent or missing pins in either end. If possible, use an alternative monitor cable that you know works. Though cable failure is rare, sometimes they can short out or otherwise become faulty.
The on-board video connector could have separated from the motherboard. If this is the case, the signal from the computer is not being processed. Connect a known working monitor, if you have one, to your computer to verify whether the message still appears. If the video returns, you will need to purchase a replacement monitor.
Bad Video Card
If you continue to receive the error message after installing a known working monitor, the problem is likely your computer's graphics processor. Depending on your computer, the processor is either be a part of the motherboard or a separately installed piece. If this is the component that has gone bad, you will need to replace the graphics processor with a new one. Make sure that the new processor you purchase is compatible with your system and, if necessary, have it professionally installed to avoid further problems.