Try opening Terminal and entering the following command to enable the App Store Debug menu:
defaults write com.apple.appstore ShowDebugMenu -bool YES
Then, quit the App Store (if it was already open), and relaunch it. You should see a new menu after the Help menu labeled Debug. Open it and select Reset Application.
If that doesn't solve your problem, you might need to flush the DNS cache, the commands are listed below.
sudo discoveryutil mdnsflushcache
Lion, Mountain Lion and Mavericks:
sudo killall -HUP mDNSResponder
sudo dscacheutil -flushcache
(to remove the debug menu from the App Store, enter the same command above used to enable it, and change the end from YES to NO.)