Cannot retrieve the missing partition of USB

Hello!
I bought a Sony MicroVault 64GB USB Drive few months ago. I subdivided it to 2 partitions with 50/50 specifications. Now I can only access only half of the available space. The other 50% cannot be accessed. I cannot see it every time I plug it in my computer. Do I need to replace it? Is there anyway that we can still recover the missing space of the usb? Is there any new software or program that I can use to restore this? Please help me!
