It also wouldn't boot off a Tiger install DVD. I figured it was dead. Luckily I have been backing up my Documents folder with rsync to an external drive on a daily basis, so I wasn't devastated.
I found my proof of purchase and rang Apple Australia to get a case number for the inevitable return to a service centre. To my surprise they helped me out.
I had upgraded the hard drive (as well as RAM). When it stopped booting, I put back the original drive but it still didn't boot off a Tiger DVD so I figured it wasn't the disk. On the phone to Apple I learned some new things:
- The original DVD that comes with your mac has extra diagnostic stuff on it that update DVDs don't.
- Hold down the D key while booting to get to hardware diagnostics on that original disk.