In your basic studies you are offered one exam per semester. You do not need to worry; you may do the exam again the following semester.
In your in-depth studies there are subjects that can only be offered with an exam once per year. You may take an oral exam the following semester.
After your second attempt you have a last chance to pass the exam. This will always be oral. The date will be arranged with the examiner at least two weeks beforehand and registered in the Examination office. If you do not pass this exam, you are no longer authorized for your studies, unless a request of cancellation is successful and you get a 2nd third attempt. You may also change the course of studies if you do not pass special exams. You may also request a change of your basic or compulsory module and get your courses so far credited as extracurricular studies or within the new module if possible.