Open a command window with administrator privileges. Run the following two commands in the following order. Note that they may return error messages, but let them run them anyway, then reboot, and see if the problem persists. Each of them may take several minutes to complete.

1. SFC /scannow
2. dism /Online /Cleanup-Image /RestoreHealth

After you do this, reboot and your start menu should work. You can cut and paste those commands, but if you do, do not including the numbering.

If, however, this happens after a clean installation of Windows, perhaps your keyboard is to blame.