My MacBook Pro 15" displays the Prohibitory Symbol on start-up. What can I do to fix it?
If you have an external drive with the OS on it, boot an hold down Option (to select an alternate boot volume) -- boot with that and see if you can deal with the primary drive.
Failing that, get an external boot drive, boot holding down Command-R, and install a fresh copy of the operating system (which should be compatible with your hardware), and then boot from that to address the primary drive.
...basically getting the data you want to save off the primary drive, and then reinstalling the OS on it.