I've had the completely-dead-system thing happen a few times quite randomly. Another board reported that worn accessory contacts in the ignition switch can do it; you might try replacing just the rear contact assembly and see what's still going on after that, as the part is quite inexpensive. Haven't done mine yet, as the symptom has been rare, but after that last bit of info I intend to.
If there's still a hot-air-only problem once the system is actually running, and you can verify the blower's operating, the monovalve is most often the problem; there's an easy and inexpensive replacement for the monovalve core.
Good luck and hope this helps!
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)