The easiest way to get your entire disk back is to use the Chrome Recovery Utility, just like you probably used to create your installer by following the below steps:
1. Open the Chrome App (from a Chrome system tray, or by installing then clicking "Launch" at the link above).
2. Select the small gear "Options" menu in the top-right and choose "Erase recovery media".
3. Insert the USB stick you want to erase and BE SURE it is the right one, since the whole disk will be erased.