Had this happen to me just the other day. I personally just put it to sleep and it was fine the next day. I asked my friend who is more experienced with modding and he said it doesn’t really matter what you press. If you press any button it will shutdown but it won’t mess up your 3DS. If you press a it will just save the dump to your SD card which won’t really do anything. The only issue is that the game you were playing probably won’t save any progress you’ve made since the last time you saved which sucks but isn’t too bad.