iMac not recognizing start-up disk

Time Machine froze my computer two days ago, and when I tried to restart, I got the lovely folder with a question mark. I reset the PRAM (Apple Support told me to wait four chimes), and everything was fine.

Today, the same thing happened. I was able to get my Mac up and running to try and repair the disk through Disk Utility, but it wouldn't repair, so I tried to run Disk Utility and repair the hard drive through the install CD. I got an error saying "invalid sibling link", and now I'm unable to get my computer to boot up at all without the install CD, and even when I do get the install CD to work, my hard drive isn't recognized.

I've tried resetting the PRAM, but nothing seems to work.

Any suggestions? I have everything backed up (thank God), so even if there is some way to just reinstall the entire system from the install CD, that'd be fine.

Here's a LINK with instructions on how to fix that error. However, if you're not comfortable with using Terminal you may be better off doing a reinstall and then restoring from Time Machine.

Also, it's possible that your hard drive is failing or getting ready to fail. It's a good idea to make sure your backups are up to date. You didn't mention if your iMac is still under warranty or not. If it is, it might be a good idea to let Apple handle this problem for you. Changing out a hard drive in an iMac is a fairly difficult task.