Does the drive appear in Disk Utility? First, choose Show All Devices from the Disk Utility > View menu. If it appears, you'll find Mount in the Disk Utility File menu. You may need to run First Aid on the disk - open Disk Utility Help > Table of Contents > Manage physical disks and volumes > Repair a storage device to learn more.
If the drive does not appear in Disk Utility, open System Information to see if it's recognized by the system port to which the drive is connected - Apple Menu > About This Mac > System Report > Hardware > USB (assuming it's connected to a USB port). If it's not recognized in System Report then you may have a bad cable/connection or a bad drive.