After installing Windows 10, there are some times your PC cannot show the Start menu. Below is the solution to solve this problem.

Windows 10 Start menu
Windows 10 Start menu
You may find a fix for the issue by checking for Windows Updates. To do this without the Start menu:
Tap or click the Internet Explorer icon.
Type update.microsoft.com into the address bar and hit Enter.
If you are prompted to run a control, click Yes.
This should bring up the Check for Updates window. Install updates.
If no update is available, or the available updates don’t resolve the issue, reboot the device. The Start menu should come back. To reboot without using the Start menu:
Hold down the Windows key and press L to Lock the device. Swipe up or press Enter to get the log in screen. Use the Power icon on the log in screen to restart the computer.
OR
Hold down the Windows key and press R to get a Command Prompt. At the prompt, type shutdown –r to reboot the device.
You may be able to correct this by re-registering the package. Run a Powershell script using the following steps:
Press Ctrl+Alt+Del. Select Task Manager.
From the File menu, select Run new task.
Type powershell, put a check mark next to Create this task with administrative privileges and tap or click OK.
Paste or type the following at the Powershell prompt, and hit Enter:
Get-appxpackage -all *shellexperience* -packagetype bundle |% {add-appxpackage -register -disabledevelopmentmode ($_.installlocation + “appxmetadataappxbundlemanifest.xml”)}