My Mac Mini (early 2009) had the exact same issue as yours. It wouldn't mount any disk, and would just spit them out.
As it turns out, my laser was dirty. I took a very fine soft cloth, similar to what you use to clean glasses and a credit card. I put some alcohol on the cloth, wrapped the credit card in it, and stuck it in the drive. Moved it back and forth a bunch to try to get any and all gunk off. The laser is pretty close to the front of the drive.
Once done, it started reading disks again. Magic.