I decided to upgrade my system so i installed Slackware 13.37. (clean install) I also have FreeBSD and Debian to some other partitions on the same computer. Unfortunately each time Slackware boots, my monitor goes to power save mode. After that it's completely dead. The previous Slackware version on the same machine was working fine. My GPU is an old nvidia 5200. I mention this cause the system falls into power save mode when the screen writes something about nouveau...
Any ideas?

PS: I can't switch off power save mode from the monitors menu.

More information would be required for a good diagnostic...

I had a similar problem once, but it was due to the inability of the distribution to auto-detect the specifications of my screen and set a proper configuration.

Watch the system logs if you can (They are mostly placed in /var/log).

Try to boot into single user mode and tell us the results.

Try to uninstall Nouveau or, at least, blacklist its modules. If Nouveau is the problem, you will be able to set another substitute (i.g: Vesa ).

If possible, you can try with other screen and see what happens. Not a permanent solution...

Add the option nomodeset to the bootloaders kernel line, this will prevent the monitor from going into powersafe-mode. Unfortunately this will also disable any hardware acceleration. I don't know if that will solve your problem, but may be a newer Mesa/Xorg/kernel will help, may be the proprietary driver (for your card version 173.14.31 is recommended) will do the job.

What is probably happening is that the either the default graphics mode or the vga= line in lilo.conf causes your video card to output a resolution that your monitor cannot auto-display. The monitor responds by going black. You can't "wake it up" because it isn't asleep, it is simply receiving a mode it cannot display.

When this happened to me I rebooted and at the lilo screen I hit tab, picked my kernel image to boot, and then added '3 vga=normal'. This booted me into a non-X session with a 'normal' vga display.

When _that_ booted successfully I rebooted, but gave it '3 vga=ask'. It then displayed a list of vga modes and asked if I wanted to scan for more. I picked different ones until I got a successful boot and display at a resolution I wanted, then changed the 'vga=???' line in lilo.conf to the mode that worked.

Reran lilo and I was done.

EDIT:
the resulting lilo screen boot line will look like 'kernel_image_name 3 vga=ask' or 'kernel_image_name 3 vga=normal' for paragraphs 2 and 3.

AFAIK, the vga= option is not supported by nouveau. If you want to force a resolution have a look at the Forcing Modes-section of the KMS page in the nouveau wiki.

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Thanks, TobiSGD, I learned something!

First of all i do not use Lilo but Debian Stable's Grub.The weird thing is that sometimes system boots. Anyway this is what i did but did not solve the problem completely.
I pressed button e and entered inside grub menu. I added "nomodeset" and after that i pressed control X. The system is able to boot now but in a lower resolution. This way i can enter Xfce but not KDE 4.