"No signal" on monitor but the pc is running fine.

Hi,

So yesterday everything was fine with my computer and monitor and then I went to sleep (turned pc and screen off as always) and when I woke up it was working okay to the point I needed to log-into my user acc then the screen went black and the monitor power button started flashing red. I did switch off the pc and monitor for couple of minutes and then turned it on again and this time it just shows "no signal" and that's it... My brother cleaned the pc for me and everything seems to be working fine etc + he thought it might be the graphics card but it's working fine on his pc. Any ideas what might be wrong? I had this problem few times in the past few weeks but switching it off for couple of minutes has normally worked but not now. Help T__T