Pages broken – "There has been a critical error on your website"

Hi guys,

A few days ago, one of our pages suddenly started showing this error:

“There has been a critical error on your website.

Learn more about debugging in WordPress.”

When I go to my WP dashboard and click on Pages, I can’t see any of the pages we have created and I see this message:
“There has been a critical error on your website. Please check your site admin email inbox for instructions.”
… except I haven’t received any admin emails to help resolve this.

I also tried creating a new header from a cloud template, and within minutes ran into the same problem. The rest of the site still seems to be functioning normally, but we don’t really want to meddle with it too much as we don’t want anything else to break – which is problematic as we’re in the middle of a complete site redesign…

Looking through this forum, other users have suggested that the problem lies with TD Composer. We have reinstalled the theme and re-updated all the plugins but this hasn’t fixed it. We’ve also tried the usual deactivation of plugins but the problem still exists. I’ve asked our IT consultant to have a look and he can’t fix it. Do you have any suggestions on where we need to look next?

Thanks