Fix Monitor 'No Signal' When PC Is Running

Quick answer

Unplug the video cable from both ends, plug it back into the correct port (GPU, not motherboard), and manually cycle the monitor's input source until it finds the signal.

Why this happens

Your monitor is basically a dumb terminal. It only knows what's plugged into it. When it says "No Signal," it means it's not getting a valid video stream on the input it's currently set to. That's it. The PC can be fully booted, Windows can be running, and the monitor still won't show anything if the signal isn't reaching the right input.

I've seen this a hundred times at the help desk. The most common trigger: someone installs a new graphics card, plugs the HDMI into the motherboard's video output by mistake, and wonders why the screen is black. The motherboard port is disabled the moment a discrete GPU is detected. Other times it's a loose DisplayPort cable, a monitor that auto-switched to the wrong input, or a GPU that hasn't fully seated in the PCIe slot.

Here's a real scenario. Last month a user brought in a Dell U2720Q connected to a custom build with an RTX 4070. Monitor said "No Signal" on DisplayPort. Turned out the DP cable was a cheap one that only carried video, no hot-plug detect line. Swapped it for a VESA-certified cable and it worked instantly. Cables matter more than people think.

Step-by-step fix

Check the monitor is actually on. Look for a power LED. If it's off, press the power button. If it's amber or blinking, the monitor is in standby and not detecting anything.
Confirm the cable is in the GPU, not the motherboard. If you have a dedicated graphics card, the video outputs on the back of the motherboard are dead. Move the cable down to the horizontal slots where the GPU lives.
Reseat both ends of the cable. Unplug from the monitor, unplug from the PC, wait 10 seconds, plug both back in. DisplayPort especially needs a firm click.
Cycle the monitor's input source. Press the input/source button on the monitor bezel. Cycle through HDMI 1, HDMI 2, DisplayPort, USB-C, VGA — whatever your model has. Stop when you see the picture appear. Some monitors have an auto-detect that fails; manual selection forces it.
Test with a different cable. If you're using DisplayPort, try an HDMI cable instead. If HDMI works and DP doesn't, the cable or the port is the problem.
Try a different port on the GPU. GPUs have multiple outputs. If HDMI 1 is dead, try HDMI 2 or DP. If none work, the GPU may not be seated.
Reseat the GPU. Power off, unplug the PC, remove the side panel. Press the GPU firmly into the PCIe slot until the retention clip clicks. Reconnect the power cables from the PSU to the GPU (6-pin or 8-pin). Boot up.

After step 4 you should see the desktop, BIOS splash, or login screen. If you still see "No Signal" after step 7, move to the alternative fixes below.

If that doesn't work

Test the monitor on another device. Plug it into a laptop or game console. If it works there, the monitor is fine and the problem is your PC.
Boot with one RAM stick. Bad RAM can prevent POST. Pull all but one stick, boot, then swap. If it posts, add sticks back one at a time.
Clear CMOS. Power off, unplug, remove the coin cell battery for 30 seconds, put it back. This resets BIOS settings that might be forcing video to the wrong output.
Check the GPU power cables. A 3080 or 4080 with only one 8-pin connected will spin fans but won't output video. Both connectors must be plugged in.
Try safe mode. If Windows loads but the screen goes black after the logo, a bad driver update is the culprit. Boot into safe mode (hold Shift while clicking Restart) and roll back the GPU driver.

Prevention

Label your cables. Seriously. A small piece of tape on each end telling you which port it goes to saves you 20 minutes of panic next time you move the desk. Also, buy VESA-certified DisplayPort cables — the $6 ones from random Amazon sellers are the number one cause of intermittent "No Signal" after a few months. And whenever you install a new GPU, plug the monitor into it before you put the side panel back on. You'll thank yourself later.

Quick reference commands

If you're on Windows and need to force a display reset, press Win + Ctrl + Shift + B. The screen will flash and the GPU driver restarts. This fixes the "No Signal" that appears after a driver crash without a reboot.

# On Linux, check if the GPU is detected:
lspci | grep -i vga

# Force X to re-detect displays:
xrandr --auto

One more thing. If your monitor has a USB-C input and you're using a laptop dock, the dock's firmware is often the problem. Update the dock firmware from the manufacturer's site. I've fixed more "No Signal" issues with a dock firmware update than I care to count.