I don't know much about F-150s, are the HVAC systems electronic or cables and vacuum lines?
If electronic, I would guess that thing took a crap (another Chevy truck I had, not the one listed in the sig had that happen and I replaced it with no further problems).
If manual, I would suspect a cloggage somewhere in the lines or possible cracked/damaged t-stat. Does temp gauge stay at same ("normal") range?