Harddisk not showing

Hello,
sir today morning i was using my pc and there were 4 partitions in my hardware in the series c,d,e,f respectively. i was downloading some softwares to my d drive andi think the capacity of the drive was almost over and in the afternoon when i came and saw there were only two partitions c and e respectively. what is the problem that my disk management is also not showing the drive which cannot be deleted because it is restricted user none of them can format or delete the partition.can u tell me whats the problem?