Your first action should be to stop departures, and any other inbound traffic should be instructed to remain with the ACC. You could use emergency separation of 500ft, or the equivalent in metres, while you resolve the situation. You should make a general broadcast that the radar has failed, and pass traffic information where possible. After lots of co-ordination with adjacent radar units, you should be able to get some spatial awareness and then use procedural separations. As a last resort, TCAS should save the day.