Assuming you have macOS managing the external drive, then within Disk Utility click on "View" and select "Show All Devices" so that the physical drives are listed in the left pane of Disk Utility. Select the physical drive and run First Aid on it. If it passes, then see if you can run First Aid on the drive Container volume. If there is no container volume, then try First Aid on the volume (or "disk2s2").
Did you encrypt the drive? Or use any proprietary software to manage the drive?
Try restarting the computer. Maybe even try an SMC reset and a PRAM reset.
If macOS is managing the drive and First Aid cannot repair it, then select the physical drive and erase it as GUID partition and MacOS Extended (Journaled).
If none of this helps, then try using another USB port, another cable, and make sure the drive is connected directly to the computer. If the drive is already connected directly to the computer, then try using a powered USB3 hub with the drive so it provides extra power to the drive if necessary (and it acts as a filter & buffer). If you have a USB-C Mac, then try using another dongle/adapter.