I opened a ticket for this, but I thought I would cross-post here in case the community has any ideas. My hosting provider had to restore our IPB installation from backup, and according to them, they do not include any "cache" directories in their backups. I'm not entirely sure this is the root cause, but it's all I have to go on for now. The main board page fails to load with the error:
Fatal error: Call to a member function paginationTemplate() on a non-object in <host info removed>/forum/admin/sources/classes/output/publicOutput.php on line 2969
(I removed the host specific info from the above error).
I am still able to log in to the ACP, but very few of the menu text, buttons, etc. can be rendered, and it's practically useless. Per the suggestion of the help desk at our hosting provider, I navigated to the Cache Management menu from the ACP, and there were two "refresh" icons at the top, although they are not labeled in the current state. Clicking the left refresh icon appeared to kick off some process, and it cycled through a list of components, I assumed rebuilding the caches. However this had no effect on the site. Now I'm out of ideas of what to try next.