Don't quote me on this but it should only be unmounted for that session. Once you reboot or something it should remount, unless you change some configurations. I don't think you can do anything if you unmount your hard drive. I'm pretty sure you would just have to reboot.

edit
What exactly would happen if I were to unmount my hard-drive while using Linux? Would I be able to re-mount it? Would the boot sector still work (I'd imagine so.) Would just my personal files be lost, and the core of Linux be remaining?

Don't want to test it out without having some idea of what'll happen...I don't want to have to do a Rescue or a Re-install..

Thanks guys. :-)
Just wanted to add something. Why would you want to do this?