My monitor says no signal

Hello,
Sometimes my monitor says no signal although I switcj my SU on... I have read ob this site Itself to hold power button or resetting the CMOS.. But it don't work. I mean sometimes it works but sometimes it don't work.. At any time when I switch it on it's as if the PC descides if it's going to power on or not.. Sometimes it powers on normally and sometimes it says no signal.. I have noticed a pattern in this.. it's like 1 day on and 2 days off.. Can anyone helf me please..