Try starting up whille holding down the Shift key until you see the spinning gear. It should start up in Safe mode. If that starts OK restart it again.
This action has no effect the restart message still appears.
Then start up from your original system disk. Put this disk in and restart holding down the C key until you see the Apple logo. When you get to the Install screen you will see at the top of the screen a Utilities menu drop down. Go the Disk Utility and Verify Disk and if OK also Repair Permissions. If disk is not OK try to repair
Have tried system disk with the same result the darken message comes over the screen as it attempts to fire up. Now I have the disc in the machine with no way of getting it out.
Restart and hold down the mouse button to get the disk out. Unplug all devices that may be causing the crash. If it still won‘t start it may be a bad drive.