Please 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.
Step 1
Rebuild the Spotlight index. If you try to search now from the magnifying-glass icon in the top right corner of the display, there will be an indication that indexing is in progress.
Step 2
Unlock the Network preference pane, if necessary, by clicking the lock icon in the lower left corner and entering your password. Cllck Advanced, open the DNS tab, and change the server addresses to the following:
8.8.8.8
8.8.4.4
That's Google DNS. Click OK, then Apply.
Step 3
Quit the App Store if it's running. Force quit if necessary.
Please triple-click anywhere in the line below on this page to select it:
defaults write -app "App Store" ShowDebugMenu YES
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Paste into the Terminal window by pressing the key combination command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.
Wait for a new line ending in a dollar sign ($) to appear below what you entered. You can then quit Terminal.
Launch the App Store. There should now be a menu labeled Debug in the menu bar at the top of the screen. From that menu, select
Reset Application
Quit again and relaunch. Test.
Optionally, remove the Debug menu by quitting the App Store and entering the command below in the same way as before:
defaults delete -app "App Store" ShowDebugMenu
See also: OS X Mavericks failed to download
