The most obvious thing that will happen is that there will be a change of state, from liquid to solid at the melting/freezing point (they are really the same thing--the temperature at which this state change occurs.) In general, as well, is that the volume decreases as temperature decreases. This is easiest to see with gasses, but also happens with liquids and solids. A major exception is water. When it freezes (ie, goes from a liquid to a solid state), its volume increases. This is because the water molecules form a crystal. As the same weight of water is now taking up more space (as ice), the density of the ice is less. You see this in action when you have ice cubes floating in your drink. If water did not have this property, there is a good chance that life would not be possible on earth, because when a body of water froze, it would do it from the bottom up, rather than the top down. It would then remain frozen, as it would be impossible for the sun to thaw it--if the surface melted a bit, it would re-freeze as the underlying layers would stay frozen.
It undergoes a change of state, from liquid to solid when it freezing, and from solid to liquid when it melts, both at 0 degree Celsius. When it freezes, all the particles settle into fixed positions and vibrate at its own position. When it melts, the particles break free from one another, but it is still closely packed, but not like solid, and slide over one another. Its volume increases when it melt.