There could be a clog in the radiator at the inlet. It happens sometimes after long periods of time running it with just plain tap water. Or possibly just a broken radiator.
Try removing the top radiator hose and checking for white buildup at the hose inlet on the radiator, also take the entire hose off and put a garden water hose though one end and turn it on just to make sure there is no blockage.
If your radiator is blocked at the inlet you may be able to remove the calcium build up carefully but you have to be sure to not get anything in the radiator because the small pieces can clog the inside really easy.
I hope this helps in some way.