Your file stills there. You only changed the filefile's path and name since you only execute mv and not rm command. Your original file must be in /fullchain.pem.
You moved the fullchain.pem file from its original path /etc/letsencrypt/live/fullchain.pem to / that's why you can't see it any more in the main.
Just execute $ mv /fullchain.pem /etc/letsencrypt/live/