Can't think of any particular procedures for complete loss of altitude info. I've come across problems with altimeters giving different reading and the pilot not being sure which one is giving the correct info. In this case cross checking with mode C data is the best solution.
If it's a case of how to get an aircraft down safely without altitude data I'd try to get the aircraft into an area without obstructions - ideally over the sea, but I'm from the UK and this is more practical than in other parts of the world - so that the pilot can descent into visual conditions and then vector or whatever toward the nearest suitable runway. This will work in all but the worst wx conditions. Obviously I'd keep the problem aircraft away from other aircraft in the area.