My iMac and iTunes no longer accepts CDs. They are rejected without mounting. Whatʼs up?

I used to be able to import CDs without any problem, running Leopard and whatever the older versions of Itunes have been. Somehow in the last year, having upgraded overa year ago to Snow Leopard, with the newer and nwer versions of iTunes dutifully installed/upgraded, my machine no longer accepts CDs but merely attempts (and attempts and attempts) to read them before spitting the disks back out, never having mounted the CD. This happens whether iTunes is running or not.