How to recover a corrupted external drive encrypted by FileVault?

I used Filevault to encrypt an external USB 2TB drive. Last week, I used the drive as usual, entering a password to unlock it to use.

Unfortunately, it didn't dismount properly. Since then I have not been able to use the drive: Disk Utility tried to repair it, but seems to have broken it even more, because now when I plug it in, Mac OS tells me that it can't recognise the drive, and prompts me to initialize / ignore / eject. Using terminal, the standard repair / verify commands I googled don't work. Please help?