Try Disk Utility first. If that doesn't solve things, you can reinstall the OS. The software is downloaded from Apple and installed automatically; no disks were provided nor needed for Mountain Lion.
I am downloading Mavericks. With my slow Internet it says it's going to take another 14 hours! I hope this works. I tried to do use just utility they got Error messages
That are not in front of me right now so I can't hold them. Crossing my fingers.
