The computer is not allowing disc to go in?

Try to see if you left a disc in the computer by booting with the (left) mouse button held down.

Try restarting the iMac then try again. Hopefully nothing has been accidentally inserted into the optical drive?

See my FAQ*

http://www.macmaps.com/cdrfailure.html

hi - force quit and then relaunch Finder; else hold Power on/off for 6 seconds to hard reboot