@Anni
When I took a look at the structure of your page(“view source”), the html tags for the logo is present but the image itself can not be found. So your issue might not be theme related.
Probable causes are:
1) The image you’re using does not exist.
2) One of your plugins might be causing the issue. Temporarily disable each to see which one is responsible.
3) This might be a folder “permission” issue with your server where a visitor is not given any access to the upload folder and its files.