Fatal error because of stargazer – who can help me, please?

Try switching to the default theme automatically by renaming the stargazer folder inside wp-content/themes and adding “-old” to the end of the folder name using FTP or whatever file management application your host provides.

Do you know how to use an FTP program? Do you know how to access your folders via your web host’s dashboard? You’ll need access to the folders to be able to rename the stargazer folder.