Applocker is a realy good feature (available only in Windows Enterprise editions), but as all security features it has some gotchas.
One of them is that if you enable Applocker on Windows 10 the Start button doesn't work anymore. If you left-click it then nothing happens. You can right-click it but that's obviously not enough.
In addition to this there are following errors logged in Applocker log in Event Viewer (Applications and Services - Microsoft - Windows - Applocker)
"No packaged apps can be executed while Exe rules are being enforced and no Packaged app rules have been configured."
To solve this you need to create a Packaged App rules in Applocker group policy. To do this follow these steps:
1. Go to Computer Configuration / Policies / Windows Settings / Security Settings / Application Control Policies / Applocker
2. Right-Click Packaged App Rules and select Create Default Rules
3. After the policy is applied to Windows 10 workstation, Start button will work again.
P.S. If you have a Windows 2008 R2 domain controller, then you will not see "Packaged app Rules" in Group Policy Management Console. You will need to create this policy from Windows Server 2012 server with GPMC.