HD have C: and D: partition, but now D: is invisible in my computer, but it show in Disk Management in control panel and drive letter D: not show. I try to assign letter but chan not change.
If you do that you will lose whatever was on it. (Hawkeye22, I apologize for following you around and disagreeing with your posts!)
If there was data on it that you want to recover, and if you did not have current backups, there are programs that will read the sectors and try to recover the files. These are usually free to scan and about $100 to unlock to recover files. If you search the forums for Disk Recovery, you will find some options. I'm kind of fond of EASEUS these days: http://www.easeus.com/partition-recovery/ , but all of them work reasonably well.