Two things to check -

1) Check the connection on the temperature sender - single wire with a black push on plg (if its still there) attached to the temp sender at the front of the block.

2) Replace the temperate sender - easy job and only cost a few pounds from any local motor factors but make sure you take your old one when you change it as they are colour coded and need to make sure you get the right one.

Other than that there isnt much else that can go wrong really from that side of things.

Beyond that you need to check the wiring to the fan, fan switch could be faulty etc etc