I get this question often, so here’s my response:
DO NOT format it. Bring it to a data recovery technician ASAP. Normally this happens when the file system or partition structure is damaged for some reason. This can happen if the drive is not ejected properly before being unplugged, or if there is a power loss, etc.
Your data is likely to still be on the drive and can most likely be recovered, but the file system is damaged, or partition table is damaged, so windows can’t find where your files are located on the drive. The raw data is there, but with no file system there’s no way to tell where one file starts and ends, or where the next file begins. If you do format the drive it will begin writing over anything on the drive, so don’t do that. What you need is called a ‘logical recovery’
To perform a logical recovery the technician may use a special utilities or equipment to read all of the raw data from the drive and create a drive image. Then they must use additional software to scan that drive image to find different file types and recover them [images, music, documents, etc. ] It may also recover anything and everything that’s ever been deleted from the drive, because the undelete process is not selective. This process can be very time consuming and complex.