Well, as long as you haven't put any new files onto the external drive they are technically still there. You might want to look into software that can recover deleted files, you should be able to get your files back but you will most likely end up with some corrupted and unusable files as well.
This might be a good place for you to start: (Lifehacker link: http://lifehacker.com/393084/how-to-recover-deleted-files-with-free-software
). Unfortunately I'm less experienced in file recovery than others here so there isn't much advice I can give you other than to try some programs out first and if that doesn't work out there are people that you can hire (expensively) to recover your data.