Images and Audio files lost after transfering from phone to SD

Hi! This is a very sad (and weird) problem, my sister has this phone and she has been using it for a while but a week ago she decided to buy a new SD card because she was running out of space. Everything was fine, I inserted the SD on it and the phone asked me if I wanted to move the files from the phone to the SD card. I accepted and the phone started moving them. I thought that it was safe to do that but I felt terrible when my sister told me that there were many missing files, in the case of the photos and images there are some of them that "appear" in the gallery but they are corrupted files and it's impossible to open them. In the case of the audio files there aren't any of them and they actually disappeared from both phone and SD card. I hope that you can help me because I don't know what to do.:crying:

Thanks in advance.

PD: sorry if my English is bad, it isn't my native language.