[ITSv40]

Without diagnostics it could be anything. Most likely a faulty micro switch somewhere or even a dry hinge or faulty catch.

I suspect from your terminology you are not in the UK and convertibles might not be in good supply where you are. If you are in the UK I would walk away and find one with a working roof - there are plenty on the market. It could be something simple or most likely an expensive head scratching thing to put right.