That is unpleasant.
As the other guys have already suggested, you should really provide some more details on the issue you are facing.
Apart from that, you can try something simple as just using a different USB cable. Also, you can attach the drive to another system and see if the issue persists.
In case it does, you can try the Ubuntu Live CD approach out. Just boot from a flash drive or a CD and see if the HDD is accessible. In case it is, my suggestion would be to back up the data stored on the drive ASAP. Here's a link: http://www.tomshardware.co.uk/forum/267999-32-recover-data-mode
Also, you can try some data recovery tool: http://www.tomshardware.co.uk/forum/id-1644496/lost-data-recovery.html
In none of these work, then your safest bet for recovering the data would be to contact a data recovery company.
Let us know how this goes,