What happened that you needed to reboot? Probably you changed something in /boot, which would cause GRUB to not be able to load your kernel. But even if you installed a new kernel without updating GRUB, you should still be able to boot. Hmm.
All I can think of offhand is to start the system with a live CD or live USB stick (or even a bootable floppy, depending on what media your server can boot from), and try doing a diagnostic from there.
Also, an essential resource, including how to make a bootable GRUB floppy:
http://www.gnu.org/software/grub/manual/
You can possibly use GRUB's user interface (see chapter 12) to boot your system without having to make a bootable medium. You just need to know on what partition your /boot is on and where the kernel is located on that partition in order to figure out what GRUB commands to create at the user interface.
NB: There is a difference between a bootable floppy that has a Linux kernel on it, and a bootable GRUB floppy, which just has GRUB info on it. The GRUB floppy needs to be able to find the kernel on your hard-drive, which isn't likely to help you now since you can't boot to write the floppy. (You should do this in the future though once your system is back up.) The bootable floppy has its own kernel and doesn't even need a hard-drive to be present to boot. It can be used to repair your master boot record (MBR) as follows (general instructions): boot using the live medium (CD, USB, floppy, etc.), mount the partition containing /boot (could/should be its own partition), and use GRUB commands to correct the problem.
Here are some resources I just got from Google for bootable floppies:
http://www.linuxlinks.com/Distributions/Floppy/
http://www.bootdisk.com/linux.htm
http://dmoz.org/Computers/Software/O.../Floppy_Sized/
For these basic operations it doesn't matter what version of Linux you're using.
Hope that doesn't confuse you. Just read the GRUB manual patiently, things will become clear.