Numbers has stopped showing up in Launchpad. How can I add it back?
Just type your application name in Launchpad's search box found near the top of every Launchpad screen. The icon for the desired App should be displayed on a new screen by itself - click and drag it to the desired location in Launchpad.
A commandline call may help as well - but be warned that it'll rebuild the Launchpad from scratch and will remove any customizations you've added:
rm ~/Library/Application\ Support/Dock/*.db; killall Dock