Power it down
Remove the power cord and leave out for 15 seconds or so
If No, insert the original OS X disc and shut down the iMac
Hold down the Option key and power it up. Keep the key down.
You should get an option of which drive to boot from.
Shut down the iMac, then restart while holding down the Shift Key. Keep it down.
iMac should start up in Safe Mode.
Shut down the iMac
Remove the keyboard and the mouse.
If it works this time, then either your mouse or keyboard is causing the problem.
You may want to try this first
I have an imac 21.5in purchased january 2010. Upgraded to OSX snow leopard. I have the original OSX (10.6.1) installation disc and snow leopard update disc. I have backed up recently onto my time machine and disconnected everything but for a wired keyboard and mouse. Now for the problem!
I think I accidentally sent the applications to the trash. Now, when I start the computer from the back button, a grey screen comes up with an alternating fast flashing of logo/file/crossed circle.
I have tried inserting the snow leopard disc while pressing C, only for it to ejected. I have also tried with the original disc.
Also tried: command + R, command + shift + V, command + S, and command + option + P + R.
What else can I try to get past the grey screen and flashing stuff, and hear the bell?