Monitor doesn't show anything

[medius]

Hello,
I've just discovered my PC "working", but with nothing on the monitor. I've left it working and went to do some stuff for about an hour. After i came back the PC was still on but the monitor wasn't shownig anything.
I've tried on-ing and off-ing the PC several times but nothing happened. I opened the PC box and turned the PC on again. I've discovered that the CPU fan doesn't move. It makes a small move at turning the PC on but then freezes again. The same thing happens when i turn the PC off.
Then i changed the power supply plug and the fan started working. I've turned on and off the PC several times since then, but the monitor stays black.

I thought this kind of behavior could show a problem with the video card, but the fan of the VC is working - i don't see any damage there.

I opened the CPU boxing and didn't find any signs of a "burned" material on CPU or surroundings. The motherboard seems to be working also.

Don't know how i can find the reason for the "black monitor" - where should i start.

Any suggestions appreciated!

Thanks in advance!