Citreon picaso cooling system puzzle????

Hi

My wife citreon told her to stop yesterday as the engine was over heating.

I've notice that the heater has not been very good lately and looking at it today the temp gauge ain't working either so I've replaced the sensor thinking it's not kicking the fan in however the new sensor is still showing no reading on the dash.

Now i'm stumped as eeven if its a nother part surely I'd still see a reading on the temp gauge???

any help please