iMac prohibitory symbol

When I tried to start my iMac this morning it first showed the Apple icon and a task bar loading at about 90%. After about 10 seconds I got the prohibitory symbol. I have shut down and re-started a couple of times, trying to reach the Disk Utility, as well as holding the option key to reach the start up disk. Nothing works and it keeps going back to the prohibitory symbol. I have been on hold with support for forty minutes. Any ideas?