I would unpot it and if the original rootball is still very dry then soak it for a while in a bowl or other container of water until it is hydrated. Then repot it having dampened the extra media first so that the whole thing is reasonably moist. If it is a peat-based medium it may be very hard to re-wet with regular watering hence the soaking and dampening. Once you've got it all damp again I would not let it dry out as much as it is now. It's hard to say how often to water a plant because it can vary but you don't want it to get so dry that the plant wilts. Of course you don't want to over-water either, which is actually more common. Watering plants correctly is not as easy as one might think
I would also take others' advice and do some cuttings as back-up in case this doesn't revive it.
