Why will my iMac will not recognize External Thunderbolt drive

I was successfully using a Western Digital 6TB external drive, and all of the sudden the iMac does not recognize it. I've tried disconnecting several times, rebooting, but not sure what the next step is. I was going to see if it's possibly the thunderbolt cord itself today, but have a feeling it's not.

Just confusing as it was working fine and now suddenly not recognizing...