Cannot find/open the documents in the hard disk

Hello,

I have a WD 1TB (My Passport) Hard disk, with many documents, files & video clip. When I copied some photos into the hard disk yesterday, it respond was quite slow (managed to transfer some photos only) and ultimately show me the message: Not responding. This message keep on running without any respond. When I pull out the Hard Disk from laptop, it prompted this: disk must be formatted, before it can be used.

I used another laptop to test & view the status by "computer management', it shows the hard disk exist, but cannot open any files.....

Any idea or solution?

Thanks