Quit the App Store of it's open.
Open the Finder. From the Finder menu bar click Go > Go to Folder
Type or copy/paste: ~/Library/Cookies
Click Go then move the com.apple.appstore.cookies and com.apple.appstore.plist files from the Cookies folder to the Trash.
Type or copy/paste: ~/Library/Preferences
Click Go then move the com.apple.appstore.plist file from the Preferences folder to the Trash.
Try the App Store.