Connected wrong power supply to 3TB WD My Book Essential - any way to fix it?

Welcome to the Community.

If you do not have a backup of the data and need to recover it, then it would be best to consult a professional Data Recovery service.

http://support.wdc.com/recovery/index.asp?wdc_lang=en

With regards to repairs, there is an experienced Board Member named fzabkar who frequents the Desktop & Mobile Drives section. Hopefully he will be able to offer some guidance.