This needs to be answered in two parts-
1. Approach- Its illegal. As such ATC will not clear you for such an approach where the applicable Navaid is U/S.
2. Departure- You can depart provided you meet the minimums criteria considering the Navaid U/S..... or an appropriate takeoff alternate has been filed etc.
Again, if the onboard system is on MEL, you cannot consider the applicable Navaid to be available... legally speaking.