i have a pc which swithches on but screen doesnt?

i have a medion md8383 desktop pc and when i switch it on i hear the fans start and my monitor doesn't come on. i have tried other monitors but the same result. i have tried other graphics cards and nothing. ram had been tested and is fine.

sometimes it comes on but whilst using after a 1 min or so the monitor goes blank but i can still hear the fans. also sometime i hear a flat line tone.

any ideas on the solution to fix problem?
many thanks,
mohit gadhok