It may be caused by poor contact between memory or graphics card and wiring.
Handling:
1. Turn off the power (if short pressing the power switch is ineffective, long press the switch until the power indicator goes out or directly unplug the power cord), turn off the power to open the chassis, plug and unplug the memory module again, and wipe the golden finger in one direction with a rubber (scraping the back of thumb and nails is also acceptable). If possible, it can be installed in different memory slots, and then turned on after installation (before operation, you need to touch the wall with both hands to release your own static electricity or wear anti-static gloves). If you can't operate it, it's easier: just tap or kick the chassis once or twice (note position: the right center of the left and right casings of the chassis without interfaces at the back; Note: don't push too hard, and of course you can't be weak. You may need to repeat the above operation. If it fails, the memory module needs to be replaced. If there are more than two memory chips, you need to test them one by one.
2. If the above operation fails, turn off the power supply and open the chassis, plug and unplug the graphics card again, and plug the data cable between the monitor and the graphics card tightly. If it still doesn't work, consider that the data cable or graphics card is damaged and replace it with a new one.