The same thing happened to me, and to make matters that much more upsetting, the person who deleted my installation, my girlfriend, was present for the installation and said she understood what you must do at each prompt. After wiping out my installation, she then said she only did what the computer told her to do.
We now have a strict rule she is not to use my Chromebook, AT ALL. An exception will be made if or when I get done printing labels spelling out exactly what to do at each prompt. The first label will read "STOP! Do not do what the computer tells you!"
So, just to be clear, your installation is COMPLETELY GONE. There is no getting it back. Please address your complaints to Google for posting instructions on each screen of developer mode which instruct no-nothing-novices to wipe out custom installations.