Hi.
If you didn't connect it anywhere else and there's no recovery key to be found, you will need to format the drive and restore from backup.
The recovery key could of course be still retrievable if you only knew who encrypted it on your machine. If you are the only person with access to your machine, it should have been you, but you say you haven't and online there's no key, so the only possible explanation to my mind is that someone played a trick on you when your machine was left unattended.