Make Sure You Switch Off Your Gopro Before Changing Batteries!

Been using my Gopro a fair bit this year without any problems, however the last time I used it I changed the battery without switching it off first,when I inserted the new battery in a message came up saying that is is repairing footage? when I got home i plugged the Gopro into the computer to download the footage it said the memory card was empty! So I took the memory card out of the Gopro and put it in a memory card adapter/reader and the footage was there,and everything had been recorded