If you have a dedicated graphics card, then make sure the monitor is connected to the graphics card and not to the motherboard video connection. For those of you who have more than one video card, you need to be careful how you connect the monitors. Make sure you check the connection at the PC and on the monitor. This is especially true for cables that don’t screw into the back of a monitor. Whether you are using VGA, DVI, HDMI or DisplayPort to connect your computer to your monitor, cables can sometimes become detached. The next easiest thing to do is to check all the cables. If none of that works, try restarting the computer by pressing the power button. Try pressing keys on the keyboard or moving the mouse. If the power is on and everything looks fine, but you’re still not getting anything on your display, the computer may have gone into standby or sleep mode and isn’t being woken up. You will have to reference the PC manufacturer manual to figure out what the beeps or status lights mean. The beeps can also be related to the motherboard, video card, etc. You can read my previous post on how to test your computer for bad memory, which can be one cause for the beeps. If the power button or one of the status lights is red or orange or if the computer beeps several times when turning on, this is indicative of an internal problem. The main thing here is to check for any warning lights or unusual beeps when you power on your computer. Now check to make sure the computer is properly turned on. Try replacing the power supply and see if that fixes the issue.

If a separate monitor works fine, then it could mean the power supply is bad on the original monitor.

Second monitor keeps going to sleep