Microsoft has acknowledged that Windows 10 sometimes has problems with the START button and developed a free repair tool. You can download and run this tool directly from Microsoft.
Over the last year our most popular pages have been How to repair the Windows 10 Start Menu when you click it, nothing happens. We built an initial page with 4 common fixes and the n another page with a much more complex, but nearly always works fix. If this tool does not work for you, try those two pages and many thousands have had success with them.