Need full details about the system and the issue, when did this start, what did you try already, full system specs, etc...? What do you mean "change the monitor", as in the resolution settings? If the monitor stays black, how are you changing things? Need to post exactly what happens from the time you turn on the computer, what you see, error messages, when they come up, can you see the BIOS screen, can you get into the BIOS, and details about when this started.