If something like this suddenly starts happening on a site that has been happily working and stable, look for changes in the host environment. Most common at present is updating the php version without telling you.

In the mean time, as the problem appears to be in the retrieval of a cache record, you could try clearing the cache. You may be able to login directly to the dashboard at yoursite.com/index.php/dashboard. If not, you should be able to use the host cPanel to empty the directory /files/cache/ (but that won't clear the block cache)