How can I fix the external hard drive?
We need a bit more information in order to help you. Specifically, is it a flash drive or a hard disc? Exactly what error do you get when you plug it in? Have you used it with this computer before, or only on other computers?
what happened exactly? is it a new hard drive?
Note: I am assuming it is a USB drive and does not show up with a drive letter in Windows Explorer.
Does it show up under Disk Management? If not, there is probably a communication error regarding the USB connection. Check to see if it works on another USB port, preferably one of the back connectors on a desktop PC which tend to be more reliable. Also check any external power source to the drive. You should also check to see if it works on another computer, if possible.
If it does show up in the list, make sure it is flagged as Online. If it isn't, right click on the drive and select Online from the context menu. That should make it available to Windows. Also, make sure it is using a Windows-recognized format (NTFS, FAT32) and it is being given a drive letter.