Heading contains weird symbols

[musicpanda]

When I want to enter a level 3 heading in my document I see - instead of numbering like 2.3.1 - just weird symbols.

How can I get rid of this and my numbering back?

Unfortunately the rules of this forum forbid me to upload a picture. My karma is too low to be allowed to explain my problem in a visual way.

[gabix]

You don’t need to upload a picture. Share the problematic file instead using any file sharing site like DropBox or whatever.