which pipe doesn't have any water?
Your thermostat has 3 pipes into/out of it. The bottom one is the feed in from the bottom of the radiator. The one off the top on an angle is the bypass hose to the water pump. At 1500rpm upwards, the suction of the waterpump opens the bypass valve to allow the coolant around the complete system. The hose coming straight out the top of the thermostat goes to the heater matrix and cylinder head. The valve in the thermostat to this pipe is the traditional temp operated valve.
If it is one of the top hoses that is empty the I would try a new thermostat first. If it is the angled hose then it could also be knackered or weak water pump.