1. Ensure that USB cable is properly seated at both ends - disconnect and connect again firmly
2. Ensure external HDD is getting external power (if there's provision for external power)
3. Is your External HDD showing in BIOS
4. Does it show in Disk Management (Right Click on Computer >> Manage >> Disk Management)
5. If HDD shows in Disk Management, is it assigned a drive letter (like E: or F: ) - if not, you need to assign a letter and restart your computer and it should be visible
Please do that and post the result here for further troubleshooting.