If you have a Linux machine, look in /sys/class/block and see if it shows up there. That'll give you a list of devices at the block device level. There should be an entry there, regardless of what you've done with partitioning or filesystems on those partitions.
If it doesn't show up there, the drive is probably having trouble at a hardware level. There are various tactics that people have tried to get a rotational drive functioning, like different temperatures, having the drive in different orientations, etc. Those might work, but if the drive is having physical problems, it might also continue to degrade.
If that's the case, your best bet, if the information is sufficiently worthwhile to you, is probably to send the drive to a data recovery company. What they'll do is use a drive with the same hardware, and in a clean room, swap the platters, and as long as it's still functional, they can image the drive at that point. IIRC you're talking something in the neighborhood of $500, though I've never needed to do this myself (backups!).
If it does show up, then you can look at whether you're getting kernel log errors when attempting to read from the drive (journalctl -k -b). If so, it might be recoverable, at least in part.
If there aren't any errors, then whatever your issue is might only be in terms of the data on the drive. My first step -- knowing nothing about how Unraid sets things up -- would probably be to look at the partition table on the drive (sudo parted <drive-device-name>). You can manually mount a partition with (mount <partition-name> <mount-point>).
EDIT: Oh, one last note. You might try swapping the cable before throwing in the towel, if you haven't already. While I doubt that this is it, and I don't think I've ever had a problem with a hard drive, a few times in my life, I've run into puzzling problems where a device isn't visible that came down to a faulty data cable. Can't hurt to try, at any rate.
