|I agree with OtheHill.|
Adding to that...
Test the hard drive with the manfacturer's diagnostics.
If you can't access the drive properly with that, you need to take the drive enclosure and it's electonic circuits out of the picture, and you'll have to take the drive out of it's case and install it in a computer and connect it ot set the bios setup so you don't boot from it, and then try the dignostics.
If you still can't access the drive with the diagnostics, the drive is toast.
If you can access the drive with the diagnostics, you run the longer test, and the hard drive itself passes without any bad sectors being found, whatever is wrong with the data on the drive can propbably be fixed, but in the worst cases you may have to wipe the data off of it and start over with fresh partitioning and formatting.
If your external drive is USB connected
If there were no bad sectors found by the diagnostics, the problem was probably caused by you not doing this at some time:
"Make sure you always click on the icon for Safely Remove Hardware in your taskbar when you have plugged in a USB connected data storage device and Stop accessing it BEFORE you remove the drive, IF the computer is running and Windows has loaded."
or by some event that happened that shut down the computer suddenly without properly shutting down Windows while the USB drive was being accessed.
If you have valuable data on the drive you can't replace and want to try fxing the problem instead....
If the drive is partitioned using FAT32, which isn'y likely...
FAT32 has two partition tables - the second one is a spare copy - in normal circumstances it's an exact clone of the first one. Some diagnostic programs, such as Norton Disk Doctor, can detect a corrupted partition table and try replacing the partition table being used (the primary one) by overwriting it with the spare copy, but if both are corrupted, that won't help.
On the other hand, something else may have been wrong.
Some diagnostic programs, including NDD, can fix some other problems too.
If the drive is using NTFS partitioning, which is usually the case if the drive is larger than 32gb, there are things you can try but it's a lot more complicated.