Excellent monitor computer and the device is excellent

The monitor works and the device also works, and the cables are new Its problem is simple: the cables don't connect the device to the monitor, maybe the cable connection port is faulty or because our electricity is weak since it's shared. Since I bought it used and it was excellent, it worked and then stopped. To be honest, its usage was light. The computer is suitable for companies and offices. Now my children have grown up and no longer need it. The cables are brand new. We bought new cables, thinking maybe the problem was with the cables, but we don't know much about computers. The problem is that the cables don't connect the device to the monitor, even though the cables are new. I couldn't fix this issue.