Q: App store won't open

Do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart or hold down the option/alt key during a restart and select Recovery Volume). Run Disk Utility Verify/Repair and Repair Permissions until you get no errors. Then re-install the Os.

OS X Recovery

OS X Recovery (2)

This worked for me. After I executed the command I was able to shut down the app store. Then I proceeded with the pending updates.

Thanks!