You need to boot your system to your Linux installation, then invoke "lilo", which will
rebuild your boot sector.
The hard part is probably the booting to your Linux installation. If you created a
bootable rescue floppy after your installation, then you can boot using it.
If you did not create a boot floppy, then you are going to need to boot using one of the
"rescue" Linux CD distributions, such as Bootable Business Card (http://www.lnx-bbc.org/
Once you have booted, you need to mount your HDD partitions under some mount
point, arranged as if you had booted normally. Then cd to the mount point for your
/ partition and invoke "chroot". If you did everything successfully to this point, you can
invoke "lilo", which will rebuild your boot sector.