Source: Apple – Support – Discussions – Can’t reset to dock defaults?
What worked for me:
delete the files com.apple.dock.plist and com.apple.dock.db from homedirectory/Library/Preferences and restart Dock. To restart dock either quit it in the activity monitor or enter the following command in terminal:
killall Dock
The dock will restart and it will be rebuilt to the default state.
The [...]
Reset the Dock on your Mac to its default configuration