Critical Error: Start menu and Cortana aren't working...

Edit: After the above steps, start menu will come back but live tiles & some metro apps won't work. You need to run the following command in an elevated power shell (same command as noted in other solutions): Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

Ok all, try this:

Unplug yourself from the internet and restart your machine.

When you've restarted, from the search bar, find 'disk cleanup' and run it.

Remove 'windows old'.

Restart your machine.

This has worked both on my computer and my wife's, both os which are running windows 10 upgrade, from a previous windows 7 operating system.

Good luck all.

I followed your idea. At the login screen, while pressing  and holding the Shift key, I clicked on the power button, chose Restart and then Windows started to work on updates and telling me no to turn off the computer. It completed the updates in two phases and then went into the legacy page where there are various choices, among them the Troubleshoot option, but I was curious to know what the updates did, so I chose "Exit and continue to Windows 10."

After a few seconds, the desktop showed up with no "Critical error." The usual icons on the taskbar slowly started to appear.  I let Windows do its thing until the hard drive light went  off.  Windows 10 has been working fine since then. :)

I am not able to run the System File Checker because the start menu does not work!  This is the error that I'm getting "Critical Error  Start menu and Cortana aren't working.  We'll try to fix it the next time you sign in."

I can't figure out how to run as an Administrator to perform the System File Checker.  I tried to run it at the command prompt and I'm getting the "need to run as an administrator" message.

I performed the other error task message at the command prompt and I got this:

Microsoft Windows [version 10.0.10240]
(c) 2015 Microsoft Corporation.  All rights reserved.

c:\WINDOWS\system32>reg query HKEY_LOCAL_MACHINE\Software\Mircosoft\IdentityStore\Providers\{D7F9888F-E3FC-49b0-9EA6-A85B5F392A4F}

HKEY_LOCAL_MACHINE\Software\Microsoft\IdentityStore\Providers\{D7F9888F-E3FC-49b0-9EA6-A85B5F392A4F}
ApPlugineDLLPath REG_SZ  MicrosoftAccountcloudAP.d11
Connected REG_DWORD  0x1
D11Path   REG_SZ  %SystemRoot%\System32\wlidprov.d11
Name      REG_SZ    MicrosftAccount
SupportedFeatures    REG_DWORD   0x1
TrustedRootContainer   REG_SZ   Windows Live ID Token Issuer

HKEY_LOCAL_MACHINE\Software\Microsoft\IdentyStore\Providers\{D7F9888F-E3FC-49b0-9EA6-A85B5F392A4F}\SettingSync

C:\Windos\system32>

If any of that helps at all.  Please advise how to get my machine back running again...like it used to before installing Windows 10.