You need to reinstall LILO (or GRUB, depeding on which one you had). To do this you'll need Linux boot disk or installation cd (as rescue disk).
If you have a boot floppy somehere, just boot from it and you should get your system. Then run 'lilo' and if it installs succcesfully you can reboot and test it (boot menu should be back).
If you have a boot cd press ctrl+alt+f2 to get a console. Then you need to mount your main Linux partition - ask your mate, should know it (it's probably hda5). To mount your main partition run
mount /dev/hda5 /mnt/lnx
(if your main partition is hda5, if not, change the value). Then
LILO should install and you caould test it after a reboot.