We've identified an issue where a plugin with malformed or missing version info can result in a non-working Plugins menu. We'll release a fix for that as soon as humanly possible in Sketch 46.1. Meanwhile, you can try disabling plugins that don't have a version number using Sketch 45 (you can download it from https://sketchapp.com/updates/#version-45-2) and then launching Sketch 46.
Hope it helps!