There are a number of bugs that can cause this. The simplest way to try and fix the issue is through Microsoft's start menu troubleshooter http://aka.ms/diag_StartMenu After it's done running you'll need to restart, and see if the issue is fixed. Also try installing Windows updates, which can repair the start menu.
There are repair options you can try, as well as, login in to a new user account, refreshing or completely resetting Windows 10. Post pack if the above repair worked or if you need additional steps to resolve the issue.