Did you watch the temp guage? If it took a really long time for the engine to heat up your thermostat may have failed. If this is the case you would have trouble getting hot air. If not there are a few other things it could be. Mostly likely the monovalve (I'm not sure, but I think your model has one) or at least the monovalve insert. I had the monovalve insert replace in my old 420SEL and got my heat back 100% right away.
1995 E420 - retired
1986 420SEL - retired