1. When you plug it in can you hear it spin up? - If not the cable might be broken, or something in the enclosure. Or worst case the drive is totally dead.
2. Check if the disk shows up in Disk Management ( search diskmgmt.msc in start)
3. Check if the disk shows up in the BIOS
Also trying it on another computer never hurts. If your USB ports are broken on your computer it won’t show up. If you don't have another computer verify the USB port is working with a USB stick or something.