Apple support article.
Three other things you can try which have worked for some people.
- Sign out of the App Store (in the Store menu)
- Quit the App Store
- In Finder - Go>Go to folder... (or Apple-shift-G)
- Paste in the following "~/Library/Caches/ "
- It will take you to a folder. Delete com.apple.appstore
- Launch the App Store. Sign in and then reclick the Mountain Lion buy. It won't charge you again.
Reset the App Store.
Try booting into the Safe Mode. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal.