Source: msdn.com

The original articles states that:

• Sometimes, after upgrading to Windows 8.1, the Store App may not start.
• It might still show the number of available updates on the Live Tile.
• Starting the app may show a progress ring but it may not finish loading.

 I had the 2nd and 3rd issue. App Store showed 15 updates on each restart and when I tried to open it I was welcomed with a message saying that it was impossible to connect to the store right now and "retry". By hitting retry, the store opened but no updates were waiting for me. And I was running Windows 8 so it applies to 8 instead of 8.1. Running the first fix (right below) fixed this issue.

• To fix this, run the following in a Command Window (CMD) to re-register the Store App:

powershell -ExecutionPolicy Unrestricted Add-AppxPackage -DisableDevelopmentMode -Register $Env:SystemRoot\WinStore\AppxManifest.XML

 

• If you get error 0x8007064A after executing this, it means that the registry key "HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppModel\Repository\Packages" does not haveSYSTEM as the owner.
• Change the owner back to SYSTEM as explained here and try again.

 

• To fix three other system apps such as PC Settings, run the following:

powershell -ExecutionPolicy Unrestricted Add-AppxPackage -DisableDevelopmentMode -Register $Env:SystemRoot\camera\AppxManifest.xml

powershell -ExecutionPolicy Unrestricted Add-AppxPackage -DisableDevelopmentMode -Register $Env:SystemRoot\FileManager\AppxManifest.xml

powershell -ExecutionPolicy Unrestricted Add-AppxPackage -DisableDevelopmentMode -Register $Env:SystemRoot\ImmersiveControlPanel\AppxManifest.xml