If your external USB HDD have disappeared from Windows 7 and it does not detect it anymore then you can download the Microsoft USB Fix It tool to repair this Windows problem.
Here you can Download the Microsoft USB Fix-It tool https://support.microsoft.com/en-us/mats/windows_usb_diagnostics
After you run the Microsoft USB Fix-It Tool then it is recommended that your restart your computer.
If your external USB drive still cannot be detected then try to unplug it an plug it back in a new USB port.
You also can run a scan with WinSysClean to make sure everything is ok with the USB drivers.