Open the Apple System Profiler by Option-clicking the Apple menu and selecting the first item.
In the left pane of the System Profiler click on USB to check if you even see the USB Adapter, dock, or enclosure listed in the right pane. If you don't see the USB Adapter, dock, or enclosure, then you have a bad cable or enclosure/dock/adapter. If you do see the adapter/dock/enclosure, then check if the physical drive is also shown. If the drive is not shown, then it may mean the drive has failed or the adapter/dock/enclosure may have partially failed.
If you are using a USB hub, then try connecting the drive directly to the computer. If you are not using a USB, then you may want to try the drive with a good powered USB3 hub with UASP support since a powered hub may supply extra power to the external drive plus the hub will act as a buffer between the drive & computer.
If you have access to another computer you can try connecting the external drive to it as an extra way of testing and confirming the problem. Even a Windows computer can be used to determine if the cable, adapter, dock, enclosure, or drive are all working. Just in case the partition table or file system is damaged you will need to check if the drive is seen by using the Windows utility "Disk Management".