Start menu disappearing after a few second or before if clicked on

Had the problem as above after years without anything significant. Right click menu on the start button continued to work just fine as did everything else as far as I could tell. Logs showed nothing useful (to me). I tried searching on here with no success but I will admit that's probably my fault. Anyway, I remembered some posts suggesting repair update as a solution for some problems so I did the using WU but nothing changed so in the end I just restored the C: drive from a Macrium backup and that sorted it but I'm left wondering what caused it. It wasn't after an update and I hadn't done anything that might have caused it. I was surprised the repair didn't correct it so I'm wondering what was different in the full restore that sorted a system problem.