EDIT: the information that pinakighosh.rhce posted looks like it should be helpful, but good luck is all i'll say (see below)
I've had this happen pretty recently too, using Kubuntu. The not really quick, but sure-fire solution that i came up with was to re-install Linux, but not changing the partition tables, just putting it right over where it used to be.
to repair Grub, but got no-where and gave up.
This actually happened to me several times in the span of two weeks. One time, directly
after reinstalling, i didn't boot into Kubuntu, but went into windoze to check out some things, and then when i rebooted to do Linux stuff, i got the error. Another time, i physically removed my HDD, switched with another to tweak some settings, and switched back. When i booted after the switch, i got the error. So, while pulling my hair our, reinstalled linux, booted into windows, downloaded Mandriva, and installed that.
Needless to say, i was not very impressed with the frequent
problems i had with Kubuntu. But things with Mandriva have been great so far