First I would check to see if you can hear the disk start spinning up when you connect its power / USB cables if applicable, some disks require only the USB connection. If you put your ear to the drive while connecting its cables you should be able to hear it start spinning up, if you are not able to then the disk is physically damaged and will need professional data recovery.
If you can hear the USB drive spinning when connected, now we can see if the disk has logged any errors. Download a Hard Drive diagnostic program such as HD Tune which does offer a free 30 day trial. Once you have the program installed be sure you have your disk plugged in. Now launch the application once the program starts it will default to computer’s internal disk, you will have to use the drop down menu to select the USB drive.
Once your external drive is selected you will want to click on the health tab (red plus), from here you will see any warnings or failures on the drive. Once you have got this far please update us with your results and we can provide a plan of action.