Going to investigate further tonight, but here is what happened:
Booted computer, got past sddm, black screen, nothing. This was MangoWC. Tried plasma wayland & x11, nothing but black screen.
Managed to get into a TTY and eventually got a message about pyenv shim (wish I had written it down!!).
So I chsh’d to zsh and all is working again (am at work ATM).
Going to nuke pyenv when I get home and try to reinstall it again. But really weird, I checked what updated yesterday, pyenv was not in it, so something got screwed.