Troubleshooting Compressor Failures
1, Inside Compressor go to Settings, to the Advanced tab, and click the Reset Queue button, then perform the Cancel Jobs process.
2, Repeat the above, but this time perform the Repair Compressor process.
3, Reboot Mac and launch Compressor.
If you still have issues, you can hold both the Cmd & Opt keys together while launching Compressor to delete the preferences files. It will NOT touch your Compressor presets, so it's safe to do.
If you still have issues, download the free FCS Remover app from Digital Rebellion to remove Compressor (I'd do FCPX and Motion at the same time, too). This removes all the system files, also, and is the safest, most comprehensive way to remove these apps. Then reboot the Mac and re-install the apps from the App Store.