WD Gold 6TB disappeared from system after 3 weeks

I bought this WD6003FRYZ for backing up large digital photo files. After working fine for about 3 weeks, now I can’t find it. Windows 7 Disk Management can no longer see it. Device Manager cannot see it. I tried booting into Safe Mode and then back to regular boot–a tip from another forum–but no luck. I also powered down the PC and double checked SATA and power cable.