The refracting prism that points the laser at the disc gets dust on it, and prevents the automatic laser focus cycle from being successful. The symptom is the drive will accept the disc, spin, make some tiny clicking noises (the focus cycle) and then spit the disc out. Take it apart, and clean the prism surfaces (and the laser lenses, since you're in there), and the drive will work again.

Though, even a clean drive will not read a CD-R.