I occasionally use falsetto/head voice in singing and more generally, I use my head voice sometimes when I'm speaking. But whenever I'm sick, my head voice goes away for literally months. Well after my chest voice feels normal, I reach for a falsetto note and just nothing is there. My vocal chords don't move at all, there's just a "friction" sound - like I'm trying to whisper.
If I really relax when trying to hit a falsetto note, then sometimes about half-way through a breath I can "break" into an actual note, and in the span of about a second the note ramps up to a normal sounding falsetto note. But then I'm out of air and trying to again hit the falsetto note with the next breath fails.
Why are my vocal chords "locked" on falsetto when my chest voice otherwise feels normal? How can I get my head voice back?