There are many types of LED display problem, here we will analyze one of the common type: part of the panels appear black. See the below image:
When the problem happened like the above picture, there are several possibility: Signal cable CAT5 or CAT6 failed, receiving card failed, the power supply failed or power cable failed. Below are the steps:
First, we should check the signal cable CAT5 or CAT6. Check if the cable connection is solid. You could try to plug it off and then plug it back. If it is still not working, then replace the cable with a new one. If still not working, then go to the next step.
Second, check the power cable. Normally each panel has several power supplies. If the power cable failed, the receiving card, fans (if there is), power supply, etc. all will stop working. So you could use the multimeter to test it. If part of the components (like part of the power supplies or fans) are working, it means the power cable is in good condition, so the problem should be the power supply. And then you could use the multimeter and find out which power supply failed. If all of components are not working, the problem should be the power cable. Plug it off and then plug it back and try. You could also use a new cable to replace the old cable and try. But if each panel only has one power supply, we are not sure if the problem is from the power supply or from the power cable. Normally, the power supply has higher potential to get damaged than power cable. So we should always check the power supply first. We could use a new power supply to replace the old one and see if the problem is from the power supply or not.
If it is not the power cable or power supply problem, then you should check the receiving card or the signal cable from the connector to receiving card. Try to replace both with new spares and test.
For this kind of problem, most of the time it is the power supply problem or the signal cable problem. Sometimes the cable quality or power supply quality doesn't seem important as they look like just small components. But in fact, they are very important. Especially for outdoor screen, the LED display face more challenge such as high temperature, rain, cold weather, etc. So the components like power supply, cables, etc. should be used high quality ones.