Dear user ,
Right click my computer then manage and then disk management .
It will display all the drives of your pc.
If your lost drive shows unallocated apace then just reallocate it...
Still the problem is not solved the go to command prompt type cmd
=> cd/
=>type disk part
Now it will display all the drives now
Type select volume 1 or2 (Ex. if ur d drive is drive 2 as shown above then type select volume 2
=> it will display drive 2 is select volume.
Now type "assign letter D"
press enter .. Now you are done..