What operating system are you using?
Win 8 and Win 10 will wake the PC for "maintenance" and not turn on the monitor.
When this happens can you wake the screen? If you can, After this happens wake the screen and open an elevated Command Prompt, by typing "CMD" in run then right click on it and select "run as administrator", in command prompt type in the following: powercfg lastwake , and hit enter. this will tell you what woke the PC last.