Often times, the problem is related to accidentally changing the language you selected for the spell checker. In this case, no German words can be recognized as correct.
It can also happen that several languages have been selected at the same time. In this case, incorrect word fragments in other languages are also recognized and are therefore no longer crossed out by the spell checker.
In the event of an error, open the spell check settings and first check that the correct language has been selected.
In Word 2013, you can access the settings via the Review tab and the fifth icon from the left.
In order to avoid possible further errors, you should always only have selected one or a maximum of two correction languages. It is best to choose your preferred language as the default.
Make sure that you have not checked the option "Do not check spelling and grammar".