I encountered this problem recently on a smaller scale. I disable iMessage for various reasons, but recently had to factory reset my iPhone, which led to iMessage being enabled. I completely forgot about it and after I remembered to disable it, I suddenly could no longer receive text messages from my friends with iPhones.

I gave Apple a call and initially the only response I could get from them was "Just turn on iMessage" and general confusion about why I had it turned off in the first place.

Eventually someone I talked to said they could fix it, and shortly after I started receiving messages again.