Fix Critical Error WordPress Page

One page on my WordPress site has suddenly stopped loading; instead it shows the notice “There has been a critical error on this website.” I have already updated WordPress core and every plugin, flushed cache, and re-saved permalinks, yet the problem persists.

I need you to:
• restore the page so its content displays normally on both desktop and mobile,
• add a button that lets users watch an activation-instructions video, and
• give me a clear, plain-English summary of what caused the failure (theme conflict, PHP fatal error, plugin clash, database issue, etc.) so I can avoid it in future.

Please work directly in the existing environment by enabling WP_DEBUG or using your preferred debugging tools, but keep the rest of the site live and unaffected. Once the fix is in place, I’ll test the page myself; if everything renders correctly and your root-cause notes make sense, we’re done.