well depends if you have a normal or low temp one and also could be stuck, to unstick it slowly heat the copper part over a flame from the stove or a heat gun, it should slowly open, but if it dosnt work at 90*c again then its defently knacered but for £7.50 for a low temp one its hardly worth taking a chance
as the fan may run but a stuck thermostat will kill your enigne
thus why i dont run one,one less thing to worry about, and the thermostat will spend its life open 99% of the time on a 5 turbo in the summer, when colder weatehr arrives ill put it back in, but the turbo keeps things warm in the colling system via the water cooled part and the oil cooler in the rad