Single post view isn't working

I have a subsite mapped as, that I recently transferred over. Something seems to have become corrupted in the process, such that the single page view doesn’t work, e.g., posts never display in their full page view, only in their excerpt. When I click on an excerpt to view the full article I get a message that there is a critical error on the website. I’m presuming its a problem with the theme, which was working fine previously but I guess could have become corrupted in the transfer. Help!