Try the following steps and check if the issue is fixed:
1. Re-seat monitor cable (VGA/DVI/HDMI) and check.
2. Reset the monitor to factory defaults (if this option is available). Run Self-test on the Monitor (if the manufacturer has provided this feature). If Monitor supports Built-In Diagnostics, run the test. If test fails; the Monitor is faulty, else follow next Step.
3. Start the computer in BIOS (System Setup) and check if the video is continuous. If yes, follow next step. Else, the problem might be with the Video Card in the machine.
4. If the computer manufacturer has provided diagnostics feature in your computer, please try running the same and check if it produces any error code.
The above steps may apply to some of the computer/monitor models, refer to the user's manual for more details.