Data Recovery from External Harddisk

Hi Guys ,
I have external harddisk which is 1TB , there was a power cut when i was operating the drive and it is corrupted. Once power is up and running , i cant see that drive in my computer . went to disk management , i can see it some times as unallocated . I have important data in that .
Could you pls tell me how to retrive data from that drive .

Thank you.