The MicroSD card is in a holder underneath the battery:
It may have come out when you changed the battery. I've got 3 sabers with this board and I found that in 2 of them the card wasn't properly inserted when I got the saber, leading to issues. The top metal piece that holds it in place slides towards the negative terminal on the battery holder until you hear it click. If it doesn't, then the metal piece can flip outward, causing the MicroSD card to not be connected anymore. That's probably what happened to your saber. Since it was working fine before you took the battery out, it had to have been in there prior to that.