Hi! I had the same shut down while startup problem. Restarted the Imac 24 (2008), holding the command+R buttons after the startup theme until some soultion options came up. One option was to restore from time machine backup. I did not have time machine turned on on this machine, so I got my macbook pro and an external HDD, made a time machine backup. When that was done, I plugged the HDD into the Imac, which recognized the system backup of the macbook pro, erased the old "broken" OSX along with the whole Mac HD, and installed all the system from the macbook pro (including softwares, browsing data etc.). After that it restarts itself and works happily. Always have an external HDD connected to your mac and have time machine turned on! It gives you great safety.