It could be the cable, monitor or GPU. First try using another cable. If your monitor happens to have more than one input try using all of them. It seems that something is going on with your monitor and the only way to find out whether your monitor has gone bad, is to test your system with another monitor. If you keep experiencing the same issues on another monitor and you have also tried other cables then your GPU is probably bad or damaged. In order to know for sure if your GPU is fine you should test it in another system. If it works there, then maybe something is happening with your PSU or motherboard (PCIe slot).