This has happened to me before, but with a metal screw on a different
part of the car. This is what I did:

I used LocTite Permanent Adhesive to glue the head back onto the
threaded "stump", and an adjustable wrench to clamp them together while
drying. You gotta let it sit for several hours, I'd recommend a whole
day if you wanna be sure. Then you can try unscrewing it again.

Some notes:
LocTite is ideal, but you can try Gorilla Glue or Krazy Glue. For
plastic screws, I don't think it matters as much. And if you can't
clamp the two parts together, It'll probably still work, but the parts
won't be as strong. So exercise caution.