You didn't say anything about specifically what machine you have, what OS version you have, are you using Time Machine, what external devices (if any) you have and so on and so forth, so I'll have to be extremely generic in my suggestions.
You might try connecting your machine to another Mac via a firewire cable and restarting your machine holding down the "T" key. This will put it in firewire disk TARGET mode, and the hard drive SHOULD show up on the other Mac as a normal external hard drive, from which you MAY be able to copy your User Folder (this is where all the non-application stuff you want to save will be located).
If the hard drive has actually FAILED, this won't work -- but if its just the boot system that's having a problem, this should work fine.
Assuming you are able to rescue your stuff, you can repair your HD using a tool like Disk Warrior ($100). You might just try booting into single-user mode (a UNIX-y environment) by holding command-s at startup and do the "fsck -fy" repair you'll see suggested when you boot into that mode, it certainly couldn't hurt and could possibly fix the issue. But the priority should be backing up your stuff because apparently you haven't been up till now.
Once you've repaired your system, learn the lesson and develop a backup strategy. Good luck to you!