try bleeding it - there is a sticky somewhere - but essentially, start it from cold with the expansion cap off, let it warm up (will take ~15mins) and massage coolant hoses. Once its hot and the t-stat is open, all hoses including radiator will be hot. Switch off and put cap back on. Oh and put dial in cabin to full hot to let water/air flow.
if its still cold then its the heat-exchanger.
James
(PS - assumes that you have coolant in it???)