My iMac early Core 2 Duo A1174 suddenly does not boot. I get the start up bong, and the apple on the grey screen, but the spinning wheel just goes forever. Nothing seems to be active - shift for safe mode, PRAM reset, C for booting from DVD, verbose mode, nothing is recognized. Resetting by the power button and with any of these key commands does nothing. Have pulled and a replaced ram. Any suggestions?