First, close the app store or itunes, etc. I fixed this by opening a terminal, then getting a super user terminal:
sudo su -
enter your password
now be careful. You are root.
Next we want to kill all the App Store related processes. This doesn't hurt anything. Let's find them using the ps command and then using the grep search utility to get down to what we want.
MacBook-Pro:~ root# ps -ef |grep store |grep -vi google
501 9284 1 0 21Dec15 ?? 0:08.90 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storeassetd
501 9349 1 0 21Dec15 ?? 0:00.73 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storeinappd
501 9353 1 0 21Dec15 ?? 0:01.91 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storedownloadd
501 9459 1 0 21Dec15 ?? 0:00.17 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storelegacy
The second column is the process ID we want. We want to use the kill command:
kill 9284 9349 9353 9459
The numbers will be different on your PC. Open up app store and it should work!
I have also read other ways - removing some verisign keys from the key chain app, and others, but this is what fixed me.