My data is wiped out after a battery service I hit my Nexus 7 with Android 4.2 on it, into a glass bottle. Somehow, internally the battery got disconnected so it didn't turn on. It was only flickering a black screen when attached to a power cord. I didn't want to fix it myself so I brought it to a certified service for mobile devices. I received it back but it had installed Android 4.4 and all of the data was wiped out. 

What did the people from the service do to my Nexus 7?
Sounds like they just replaced the device and gave you a new/refurbished one. Your original device will be fixed (if possible), and then it will be wiped and turned into a refurbished unit.

If you didn't back up any of your data, it is gone. Hopefully you did; I'm pretty sure anytime you send in a device for service, they warn you that it could be completely wiped.