i dont have any idea bout the thermal past..what is thermal past?u mean v need to apply some kind of paste on the processor before placing the fan on it?
Yes, if you remove the heatsink, the thermal material needs to be re-applied. You may have noticed a white/grayish substance on the chip surface and heatsink when removing it. That is the thermal material. It ensureas that their are no air gaps between CPU and heatsink, since air is a very poor conductor of heat. No paste means that the CPU will overheat in a very short amount of time.
I suggest you purchase some paste and then read a good guide on how to apply it. I can talk you through the steps as well if you like.