Hi Raymond,
You can try the below mentioned methods in order to resolve the problem.

Method 1: Rebuild Cortana settings

1. Open Windows PowerShell as Administrator
   1. Right click Start button
   2. Choose Command prompt (Admin)
   3. Key in Powershell in the black window and hit Enter
2. Paste the following command in the Administrator: Windows PowerShell window and press Enter key:

   Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

3. Wait for PowerShell to execute and complete the command. Ignore the few errors (in red color) that may pop up.
4. When it finishes, try hitting Start and hopefully it’ll start working.

Method 2. Kill and restart the Cortana process
If you find that Cortana is not working properly, the best thing to do would be to kill Cortana process and restart the process via Task Manager. If it was a minor runtime error Cortana would just take a restart to fix it.

Method 3. Disable Antivirus software
Some security software are known to create this issue – Avast for instance. Disable it and see if it makes the problem go away. If your security software is the culprit, make sure you are using its latest version, else you may want to change it.

Method 4. Create a new User Account
The problem might be associated with your Microsoft credentials. Try creating a new username and password and see if you can fix the Cortana.

Just follow the below steps to do so:

1. Press Windows logo key Windows logo key + R, type ms-settings:, and then select OK. This opens Settings.
2. Select Accounts > Sign in with a local account instead.
3. Type your Microsoft account password and select Next.
4. Choose a new account name, password, and password hint, and then select Finish and sign out.

Create the new administrator account:

1. Press Windows logo key Windows logo key + R, type ms-settings:, and then select OK.
2. Select Accounts > Family & other people (or Other people, if you’re using Windows 10 Enterprise).
3. Under Other people, select Add someone else to this PC.
4. On Windows 10 Home and Windows 10 Professional, provide a name for the user and a password, and then select Next. On Windows 10 Enterprise, select I don’t have this person’s sign-in information, and then selectAdd a user without a Microsoft account.
5. Set a user name, password, and password hint. Then select Next -> Finish.

Sign out of your account and then sign in to your new account. If you can open Cortana or the Start menu, move your personal data and files to the new account.

I hope this helps in fixing the problem.

Regards,
Lokesh