i have a problem with debian 5 lenny upon rebooting it restart and then my monitor get out of frequency what should be the problem with that please help me solve

why are you using Lenny? It reached EOL back in February and was removed from the Debian repos.

using for our office sir how couldi fix it

People tend to be reluctant to spend time helping others fix an unsupported system which you will probably have to upgrade anyway... thus the whole exercise will have been a complete waste of time and effort for both you and the person(s) trying to help you.

People are also reluctant to help those who provide next to no information about the problem...

When booting a typical Debian system boots one usually see the following:

1) BIOS POST screen (memory test, hard disk detection, etc)

2) Boot loader screen (usually grub, possibly lilo) displaying a list of kernel images to boot - e.g. in your case probably 2.6.26 listed twice).

3) If you hit enter or let it time out, the first image will be booted and the next thing you will see are boot messages.

4) Next depending on how your system is configured you will see a console log in prompt or more likely a display manager login window.

5) After logging in you will be presented with a desktop environment or window manager desktop screen.

So the question is: At which point does your monitor go to sleep?

in the screen of choosing 2 os one is for single user then it restart then the monitor says out of frequency

Get yourself a copy of Squeeze and start again. As already pointed out Lenny is no longer supported. By using it you are exposing your office to cyber security risks that you need not expose it to.

im sorry for that right now my boss are not aware of that it seems i cant change to debian 6

Select the normal user mode (not single user mode) and hit the letter "e"

Add vga=769 to the end of the kernel line, if that doesn't work try vga=0 instead.

If either of those works, then there is probably something wrong with your grub configuration.