On your system partition (not in your user/home folder) go to Library > Application Support and move the folder “Propellerhead Software” to the Desktop.
Still in Application Support delete the file “ReWire”.
Start Reason stand-alone. The missing files will be restored.
Quit Reason.
Next time you start Cubase Reason should be again available as a ReWire device.
If you have other ReWire slaves in use there might be corresponding ReWire files in the Propellerhead Software folder on your Desktop. It might be necessary to move them to the newly created Propellerhead Software folder in Application Support to make sure that the other slaves will work, too. Otherwise, you can simply delete the folder on the Desktop.