What happens if the bus, the S-Bahn, the U-Bahn or the train stop running and I cannot get to work?

It is the responsibility of the employee to arrive at work on time. This means that as an employee, you have to make sure that you get to your workplace. Your employer does not have to pay you for the days that you do not come to work because of these circumstances.

If you are unable to reach your work place due to failure of public transportation, it might make sense to talk to your employer. Perhaps a suitable solution can be found (e.g. home office). You can also read the answer to the question: "Am I entitled to do home office, i.e. to work from home in the current situation?".