Well Grub is boot loader for Linux (The thing that tells it how, where and what to start up). Grub may be poorly (techie term, I don't think) or the hard drive may be knackered.
You could try booting from the linux disk and see if there is an option to repair the installation.
Which Linux is it? I'm sure there will be lots of in depth ways to fix it which someone who has played with Linux will be able to describe for you.
They'll need as many details as possible though
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