Recent Versions
Window -> Restore Default Layout
(Thanks to Seven4X’s answer)
Older Versions
You can simply delete the whole configuration folder ${user.home}/.IntelliJIdea60/config while IntelliJ IDEA is not running. Next time it restarts, everything is restored from the default settings.
It depends on the OS:
https://intellij-support.jetbrains.com/entries/23358108