Dumb question but did you pull the knob and use one of the other ones to try and turn it? I've noticed that 3-zone control on mine would get harder to turn when it sat and ended up with a cracked knob that would move but not turn the metal post in the HVAC unit all the way.
Pull the knob and see if you can move the post with pliers and get it to change direction. If that doesn't work then I'd think either a line is messed up or the HVAC unit is leaking vac.
IMHO if it doesn't default to defrost then I wouldn't think it's a vac problem as IIRC a loss of vacuum would indeed put it on defrost.
