How do I reinstall pre-loaded applications?

[AC Question]

My Galaxy Nexus is rooted and I uninstalled the app that allows Calendar notifications. Now Google Calendar is not working. I have tried restarting the phone, and uninstalled GCalendar and installed again from the Play Store. I did not expect a result, because I basically deleted the system app that allows Calendar notifications. Now I really need help to either get a new copy of that app or some other solution.