Mac app store is completely blank...

That doesn't really sound like an issue with the app, but an issue with your connection to the iTunes/Mac App Store server(s),

others lately have resolved Mac App Store issues by using the Google Public DNS service.

Late 2012 Mac mini, OS X Yosemite 10.10.5; Apple Watch, 38 mm silver AL, watchOS 1.0.1; iPad Air 2 & iPhone 6+, iOS 8.4.1

Test after taking each of the following steps that you haven't already tried. Stop when the problem is resolved. Back up all data before making any changes.

Sometimes the cause of the problem may be an outage in Apple's network, in which case none of the steps below will have any effect. You just have to wait for the outage to be fixed. That should rarely, if ever, take more than a few hours.

Step 1

Sign in to the App Store by selecting Sign In from the Store menu. If you're already signed in, sign out and then sign in again.

Step 2

Log out or restart the computer.

Step 3

Remove or disable "LittleSnitch" or any similar third-party software that blocks outgoing network connections according to the developer's instructions.

Step 4

Start up in safe mode and log in to the account with the problem.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

After testing, restart as usual (not in safe mode.)

Step 5

If possible, connect to a different network and test.

Step 6

There are reports that resetting the PRAM may solve the problem in some cases. I can't confirm.