coolant gauge inop.

someone help me please, i got a problem with my temperture gauge. Just brought the car and the temperture gauge is not working at all. I drove the car along time and the heaters is hot but gauge not working, has anyone had this problem or does anyone know what the problem is???? P.s i have now changed the sensor by the thermostat and still not working, i have also checked the connections at the sensor and i got a earth but no feed to the switch? could it be a realy fault?

found the fault, it was a thermostat housing. It costs around 200 pounds but it was under warrenty so garage payed lol