Nope, neither of those worked. But thanks for the suggestions!
If I right click on Like and open it a new window, it asks me if I really want to Like this post, and if I click on Submit, it takes me back to the thread, but the post is not Liked and it says the user has 0 liked posts.
The only thing I've been doing recently is changing the color palette. Though I'm not sure whether this ever worked on my installation - I can't find any existing Likes at all for any of my users. I did import a vb 3.8 board into XF.
found the problem - permissions changed themselves
Administrative changed itself to DENY, I changed it manually to ALLOW
Registered was set at DEFAULT but I changed it to ALLOW and "Like" link reappeared under posts for registered users