1) Re-evaluate the project and in particular its business case: IF it is still viable, close it in an orderly way and start another project; in doing so, make sure a product is going to be released.
2) If people want to leave (even if projects end quite regularly), I would say there is something wrong over there: a general overhaul of your practices is due. Try a poll amongst your people: discover what they deem wrong, for something must be wrong. Another possibility: there are no challenges over there, i.e., your projects are too “ordinary”, i.e., there is little left of the project environment (provided it was there to begin with).
A good Project Management methodology like PRINCE2 helps you with point 1, provided you follow at least the basic principles.
As for point 2, Project Management needs a good project environment, which should never be forgotten.