Gutenberg doesn’t show up when I edit a Page or Post

This morning when I tried to edit a page I got the classic editor and couldn’t switch to blocks. So I disabled the classic editor but still the Gutenberg blocks don’t show. What might be wrong? I haven’t updated anything today.
WordPress 5.4.2
Edit: I installed Health Check & Troubleshooting and discovered that there is something wrong with my theme. I’ll have to find out what.

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