What could cause my boiler to provide hot water, but not heating?
The room thermostat does not control the boiler directly. Instead, it controls a circulating pump that draws heat from the boiler and delivers it to the radiator(s) in the room. It is the removal of heat that subsequently causes the boiler to fire, when its thermostat indicates that its internal temperature has dropped because of the heat removal.

It sounds like either the circulating pump or the relay that controls it has gone bad and needs to be replaced.