Well error 18 is
18 : Selected cylinder exceeds maximum supported by BIOS
and that is not likely to happen especially if it worked before, but there is always a small chance that some setting is way off.
Only if it's an old laptop would you get this error at all
And laptops have very few settings in BIOS but you could (try to) open BIOS and use the option Reset to defaults or similar if it exists (and sadly I doubt that)
You could use the live CD to repair Grub or perhaps better download Supergrub (google for URL) it's only some 20 MB and creates a bootable repair CD
Don't fix it if it ain't broken, don't break it if you can't fix it