Coolant temp gauge not working? - Third and Fourth Generation VFR's

I changed the coolant in my 96 the other day, replaced it all the way up to the top of the radiator. Let it idle for about 15 minutes to circulate through the system.  However, the coolant temperature gauge isn't working now, it worked prior to changing the coolant. I don't think it is something stupid like knocking a wire loose, ideas?

could be an air block at the sender
loose off the sender and see if some water leaks past the thread then tighten it up again
just a thought.or short out the wire from the sender and the gauge should read full hot.

Where is the sender? By sender do you mean coolant sensor? Sorry for the stupid questions.

I had a similar problem. When replacing the sender which is located in the thermostat housing, I noticed that the housing should have been attached to the engine body with one bolt which was missing. The housing was supported by the rubber hoses so there was no ground connection from the sender to the body. After putting the bolt on place the gauge started to work.