Shit! What do I do now?

Besides the obvious recommendations such as EasyRecovery Pro and TestDisk, I can't think of much. Don't get your hopes up, though, because these will probably not be able to recover anything, since this disk conversion is much more severe than a simple drive format.

Makes me wonder why you had to active this drive in the first place, though. I reckon your drive was using a wrong disk type from the start so next time you use an external drive, make sure you have set it up properly. Even though it might sound reasonable, an external disk should not be of the dynamic type from my knowledge.

Jam it back in, in the dark.