That sucks. if the recovery key doesn't work, and you don't know the password, you're out of luck. The only thing you can do is try to remember the password, or check if you typed the recovery key over properly.
Otherwise, that data is gone...
If you can't figure it out, I'm sorry for your loss.
So, you've encrypted the drive, it had some errors but you used it anyways for a few days/weeks/months right?
Odd that it wouldn't accept the key anymore, usually only means one thing: It has a different password and thus the key has changed. Between that or you are finally seeing the result of the errors.
There's not really much you can do except take the original data (assuming the external disk was a backup disk), and start all over. Any data currently on the drive is lost to you, unless you can figure out the right password. Are you sure you remembered it right?