How to solve memory allocation problem in cuda??

Hi there, I have been working with CUDA for quite a long now and have implemented quite a few programs. But for the first time, I am facing a difficulty with an error called “Fatal error. Memory allocation cannot be possible”. I have searched in the internet and couldn’t find any satisfactory solution. Here I would like to mention that I am using CUDA 6 version, Linux 64 bit operating system with 64 GB Ram support and 2TB hard disk support. I have checked memory status and it shows only 3% is in use. I have used CUDAMalloc function to allocate memory. But still this problem arises. So, anybody if could help me out in this regard will be very much helpful. Thanks in advance.