Oh my, did you type 'rm -rf / ' ? That thing's legendary
I guess if you had the inspiration to immediately shut down the device a lot of things on the FAT partition can be recovered. Just connect it to a PC and run data recovery software on it. Results may vary
Now, i can hear the angry mob closing in, chanting "stay away from terminal!". My advice is not to listen to them. Just learn the lessons from this mistake. Before using a command, try to find out what it does. Just googling will pretty much tell you. If the command sounds potentially harmful ("removes files"), make sure you copy it exactly and from a trusted source. Make sure the command is not under "don't do this".
Making a backup and keeping it in the device is like making a copy of the key and putting it on the same keychain. For best results, the backup should be kept secure somewhere else.
Now, i can hear the chanting hordes even closer. Remember, i've been using linux for years and last month nearly erased my HDD in a feat of sheer stupidity. Don't lose faith because of this
Good luck!