The problem is pygbag. You need to re-upload the files using the latest version of pygbag, and you may need to use a newer version of Python if applicable.

The best thing to do is force a pygbag update with this command (lasted version is  0.9.2):

pip install pygbag --user --upgrade

Then you need to repackage the game for the web and re-upload it to Itch.

Why does this happen? I don't have the technical details, but what I've seen is that the WASM interpreter is downloaded from GitHub when you run the game and isn't included in the files you upload. Therefore, if the pygbag library updates and stops supporting an older version (for example, 0.8), all games packaged with that version will stop working, even if you haven't touched anything.

Edit: I'm telling you from memory, but I think pygbag 0.9 requires at least Python 3.12 or higher.