If after installing the September 2019 update the Start menu and search aren't working, you can try this workaround to mitigate the problem until it's fixed.
It happened again.
On September 10, Microsoft began the rollout of the update KB4515384 to devices running the Windows 10 May 2019 Update (version 1903) to address several issues, including the high CPU usage due to a problem with SearchUI.exe service. However, even though processors don't seem to spike anymore, the update now appears to be causing other complications with the Start menu and search functionalities.
Full story from the WindowsCentral blog...