Problem with external harddisk

Hello,

I have a external harddisk and use it for few years.
At first, I still can detect it as usual, but when I plug it out and plug in again, it cannot be detected.
The Disk Management can detect it, but it show unknown and not initialized. When i right click it, the 'Initialized disk' option is disabled.
I am using window 7 professional. and I just change a brand new case for my harddisk.