the Youtube block is a core embed block, and the issue is not theme related.
Simplest fix is to remove the block, and create your own embed code on this site:
We seen a few reports across WordPress forums where the YT Embed Block just stops working.
Sometimes you simply need to update the URL for the video but others have reported this doesn’t always work.
But the embed method I provided has never been an issue and as its just the one video it was the logic option ๐