external hard disk not getting detected

my brother had a seagate external hard disk. and for some reasons, it was not getting detected in any of the systems. i broke the casing and fixed the disk in an external casing and it was working fine. and after some days, the casing broke and I had to travel that day and kept the disk in my laptop bag. i kept it in a plastic cover. and when i tried to get a new casing for the hard disk, its not getting detected. tried many cases, but were of no use. please help me how to get the disk working. all my works and collection of life time are in the disk.