Make sure your USB adapter drivers are functioning well and up to
Is the external drive working fine in another system? If you've not tried it, please do.
Also try to see if another external hard drive works fine on your system.
If another external drive works fine on your system, then the problem is with your external hard drive. Your external hdd is damaged.
Get a data recovery software such as EaseUS recovery tool by downloading and installing it, (you can use the trial version), use it to open the external drive, you would see your files there. Recover what you want by saving them in another drive.
If another external hard drive doesn't work fine on your system, then go to Google, search for the USB drivers for your system brand and model. Download it.
Go to device manager, under USB, uninstall the USB adapters.
Then go back to the USB driver you just downloaded, install it.
Restart your computer.
Test your external hard drive on it.