I just fixed quite a difficult case. As it often is, the situation was made murkier by there being multiple issues. The main problem, though, was one I’d seen before: the Windows 10 start button stops working (except for right-clicks, which is useful), taking with it Cortana and other task bar located features.
On the previous occasion I was asked to fix this, it succumbed to the command line procedure described in this PC Advisor article. This time, though, that wasn’t enough. So I tried renaming the TileDataLayer folder as recommended in this video but though that basically did the trick, the start menu was left looking very sparse indeed, missing not only program buttons and tiles but even Settings. Luckily, the fix for this situation is in the video‘s comments: create two temporary users, login as one then copy that folder from the other to the “real” user account. (Then of course switch back to that and delete the two temp accounts.) Job done!