lods of reasons......as FT suggested or, lack of coolant, heater matrix blocked, heater switch siezed and not operating the valve, faulty water pump. hope that helps
If it`s about W reg, 2000, then it could be either of the ludicrously over-engineered controls: the one on the dash or the one below the wipers in the engine bay. They`re both electronic instead of the old trusty cable operated type. Common problem apparently, & not cheap to cure.