If the RAM originally fit and worked properly then it most definitely will fit again. There are tabs that need to be pushed down prior to installation on the motherboard, the RAM will only go in one direction (make sure the notch fits over the part that stick up towards the bottom center). Finally make sure the RAM is perfectly level and that the ends are in the sliding grooves that are in the tabs. You will need a fair bit of force to get it in, additionally the tabs should raise up as it goes in to a point where they lock it into place (raise piece into end grooves of RAM).
The biggest keys are keeping it level and correctly orientated. RAM was designed this way so that techs could quickly and easily add RAM.
If it still won't fit then something has been damaged and is preventing it from going.