It is possible that the reason lies in the faulty USB controller. Connect your hard drive to a different USB port. It often happens that the foreign carrier does not start because of a lack of power, especially if you use the connectors on the front panel.
Try replacing the cable used for the connection. Over time it can cause damage leading to lack of connection. If there are two connectors, both must be connected.
Sometimes the reason lies in the computer's power supply. If you use many peripheral devices, the power to connect an external HDD may not be enough. Disconnect the piece of equipment.
The external media may not be recognized by the system, due to problems with drivers. Check to see if the disk in "device Manager". You can open it by running the command "start" — "control Panel" — "device Manager".
Tap the "Disk drives". You will see a list of all hard drives. Click the row with the name of the external HDD, right-click and select "Update driver".
It is also recommended to update the drivers for the USB controllers. If the automatic search is not successful, try to download driver from manufacturer's website.
It happens that the operating system assigns the external bearer of the letter, which had previously been assigned to a mapped drive. Run the command "control Panel" — "administrative tools" — "computer Management" — "disk Management".
Right-click the mouse on the string with the name of the external HDD from the drop down menu select "Change drive letter or drive path". Assign any letter not yet used in the system and click OK.
If the system has not detected a new hard drive, it is possible that it is not formatted. Open the "disk Management", right-click the mouse on the external drive and select "new simple volume".
Perhaps your computer does not support USB in the BIOS. In this case, the computer does not see not only an external drive, but any of the USB devices. You have to go into the BIOS (how to do it, you can read the instructions to your motherboard) and enable USB support.
If none of this helped, and Winchester recorded important information, you will have to contact the service center. It should be borne in mind that the cost of repair may be comparable to the cost of a new external disk.