How to fix my Paloma PH5?

The Paloma on our boat was left un-used for several months and we have been told that the pipes may have frozen up. Not sure that is correct because the pipes going to the Paloma are plastic. Anyway after igniting the pilot light, turned on the tap to fire up the boiler but instead the water just poured out of the bottom of the heater. What is it that is likely to have frozen/broken and how to I go about fixiing it