Blank Screen Syndrome but starts up and works fine?

I have an annoying problem which i'm hoping isn't too serious...yet

my computer seems to work perfectly fine but every now and then (once a day or once every other day) i'll come back to it and the screen will be blank.

all lights are on, computer seems perfectly happy but just a blank screen.

if you move the mouse the screen does not come back on. Control-Alt-Delete several times does not make the computer reboot.

Only option is to switch the comp off and back on, which i know isn't a good thing to do.

Computer reboots and everything is back to normal and will probably stay normal for the rest of the day or maybe til tomorrow then for no apparent reason i will return at some point and the screen will be blank again.

Any help or guidance would be great.

PS. i've never opened up a computer before just in case this is what you are going to advise me to do

Is this a new problem? Were things working OK previously? Any new hardware and or software?

If this is a new problem try a System Restore to a point prior to the problem:

start > All Programs > Accessories > System Tools > System Restore


Also check your Power settings via > start > Control Panel > Display > Screen Saver > Power


What is the exact make and model of the computer?