My visa will expire during my stay in Germany, what do I have to do?

You have to apply for new visa before the expiration of your visa because without that you will not be able to visit or travel in any Schengen states. In case if you have any emergency such as medical treatment then you have to contact the authorities (Ausländerbehörde)   so that they can extend your visa before it expires.  You should avoid to stay further if your visa has expired because it is illegal and you will be fined or will not be allowed to visit Germany in future.