Sometime in the last day or two LJ made some code changes that finally broke the old xColibur
site scheme. This mostly manifests in comments not loading on non-journal styled entry pages, but some menus appear to be broken as well. People have been writing in to Support asking about this, and have been individually answered, but there hasn't been a general announcement yet.
Support staff have been replying that LJ is aware of the issue, but given that xColibur was officially deprecated a decade ago there's no timetable, nor even a guarantee, that the issue will be resolved. They are recommending that people switch to one of the modern site schemes (Horizon, Vertigo, or Lanzelot) in order to be able to read comments again. Switching to the new site design would also fix the issue.
Honestly I think it's good advice. LJ's policy over the last three owners has been to not remove old features (until they get too broken and/or unused) but not to fix them either when something breaks. We've seen a bit of that over the last year with the removal of some of the very oldest journal layouts and I think it can go either way whether LJ fixes xColibur or removes it entirely.
If you really don't want to switch there is a really hacky way to keep using xColibur most of the time and still being able to see comments. Copy the following code into a new bookmark in your browser and you'll create a bookmarklet that will reload the current page in the Vertigo site scheme. It's limited in that it will only work on entire entry pages, not when linked to specific comments, but it's better than nothing.
javascript:location.search+=(location.search?'&':'?')+'usescheme=vertigo'