How did you check for syntax errors? Manually or did you look for syntax warnings in the logs or load the files in VSCode?
When you restart OH what do you see in the logs near the beginning where it is loading the .items files and .sitemap files?
With the limited information provided I’d guess that you accidentally deleted the ] or in some other way messed up the syntax on that .items file so those Items are not being loaded any more meaning there are no Items in your Groups on your sitemap anymore.
But like Christoph asks, we need the logs, sitemap file and relevant .items files. Please How to use code fences.