Hello there,
There are a lot of different reasons for an HDD to stop showing. A few things you can try are:
Try using a different USB cable, make sure it is as short as the original one, or even shorter. This is important it terms of providing enough power for the drive to function properly.
Try connecting it to different USB ports.
Try connecting it to a different PC.
Sometimes the drive can misbehave due to insufficient power even if it appears to be spinning make sure you connect the HDD directly to the USB port and not through a USB hub.
Check if you can see the HDD in Windows Device Manager and in Disk Management. Good luck.