iMac won’t finish startup.

My iMac turns on, then begins starting up, but then ends with a blank screen. It won’t start in Safe Mode either. It just goes to the same blank screen. It was fine yesterday but woke up this morning to hear the fan running very loud but the screen was black. I disconnected the power (no other way to turn off), and started it up just fine. Then this afternoon, the screen suddenly changed to an almost white screen and was stuck there. Disconnected power again and now when I turn on, the apple logo appears, it shows the progress bar under the logo, then it comes to a gray screen where it has been ever since. Nothing I try works now. Even booting up to get into safe mode still stops at a totally grey screen. Any suggestions?