Windows 10 TP: Menu Start doesn't appear anymore, and Cortana search is not working

Solution 1:

Did you manually edit the Windows registry and the policy manager? It's not recommended to do so at this point because settings are being removed and added automatically, and there may be conflicts.

Create a new account to see if it works normally. If so, replace your current account with it. If it doesn't, you can refresh your install to restore it to the original preview status. It keeps all your files and Store apps, but removes traditional programs you may have installed.

Remember that you're using it at your own risk. Don't use test releases of software for your important tasks.

Solution 2:

Run PowerShell as admin and type this command:

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