i have recently installed mandrake-9.0 and initially all was well. however my graphic interface stopped working. my display switches off and i can't do much except Ctrl+Alt+Backspace combination.
where could the problem come from?

thank you

A bit more detail:
When your display switches off, does it just go black, or is there anything at all on the screen?
Does it happen when it sits idle, or are you using it?
Ctrl+Alt+Backspace should take you back to the command line. Does the screen show any error messages? Are you then able to use Ctrl+Alt+F7 or maybe just Alt+F7 to get the GUI to come back up?

I made a mistake. Actually, if I am on runlevel 5 all is fine. Except that display goes off for a about 20 secs. However, if I am in runlevel 3 and open X with startx then my display dies..
1) Instead of starting KDE or something it freezes the PC and I can't do _nothing_ except ctrl+alt+backsp (not even ctrl+alt+del). But before it dies dispay says "No Signal". And then nothing.
2) It happens when I start X. Nothing like that (screensaver or something) is supposed to happen.
3) ctrl+alt+backsp RESTARTS the computer, which is strange as it is supposed to shut down the x-server.
4) Screen shows nothing. Mouse and keyboard don't word (I suspect, because I don't see what's going on on the screen)
5) ctrl+alt+F7 (F1, F2 ...) do not work too.
P.S. Basically, I get a feeling that computer is off, except that it makes some noise from the processing unit... I wouldn't make so much hassle out of it, but I am about to start learning Linux and I feel that my knowledge of X is next nothing..
Thank you!

Going just by the "no signal" message, I think you should check the cable from your monitor and make sure it is not loose or something.
I guess that this might be caused by having incorrect settings (horiz/vert) for the monitor, but then it should not have worked to start with.
The restart when pressing Ctrl-Alt-Backspace is certainly not right, and I do not know what might cause this.
You said that Mandrake was a recent install, and I assume the previous OS worked correctly. Are you by any chance dual booting, and able to see what is happening with the other OS?

what does your
/var/log/XFree86.0.log
say?