Restore USB boot record solution needed?

Hi All
I have a USB drive of 300 GB. When I connect a USB drive to my computer, then windows dialog box prompts me that, the drive is not formatted. Using a diagnostic tool I found out MBR of my external hard disk is corrupted. The data still present in my drive.
Is there any way out to fix this issue? Please suggest me what to do for the restores USB boot record.
Thanks.
