Since the person at the camera shop would not explain what he did, it is likely that they did not know what they did.

It is possible that the camera has a reset button or reset proceedure. Some cameras do have a reset button. Since Nikon does not publish an online user's manual, I can not tell if it does or does not.

Check your user's manual and see if you can find any mention of a reset.

While I was on the Nikon site I noticed that there have been several Firmware updates for that camera.
I suggest you get the latest update (firmware 1.8) and update your firmware.

Click on Firware Updates
In the Search Text window - type in - 5000
Then click the Search Button.
You will see 1.8 updates - choose Windows or Macintosh.