Imac 20 inch 2009 OSX 10.9.5
You can try the following commands to fix that.


Open the Terminal application (located in /Applications/Utilities)
Type in the following commands:

sudo rm ~/Library/Preferences/com.apple.appstore.plist

sudo rm ~/Library/Preferences/com.apple.storeagent.plist
Type in your password when prompted, (your password won't appear when you type it, this is perfectly normal).


Now the Mac App Store should work again.