This procedure will reset the state of LaunchPad to its default. Any customization that you did will be removed.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Library/Application Support/Dock
Right-click or control-click the highlighted line and select
Services ▹ Open
from the contextual menu.* A folder should open. Inside it are one or more files with long names ending in ".db". Move those files to the Desktop, leaving the folder open. Log out, log back in, and test. If there's no change, put the files you moved back where they were, overwriting the ones that may have been created in their place. Then log out and in again. Otherwise, delete the files you moved.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.