After upgrade 10.6, disk doesn't boot anymore

I Installed 10.6.3 on my iMac, 20", 2009 which was still running under 10.5, from DVD, in order to upgrade to Yosemite.

Installation went through fine. Then I shut down and restart. The iMac does not start anymore, and a folder icon keeps blinking. My iMac doesn't boot anymore. Even worst, when I launch the Disk Utility on the install DVD, it doesn't recognize my HD !