Complete strip down called for here Sue.
That's plumber talk sorry, what type of mixer is it? two taps joined together or a variable mxer where you turn the middle on and the outside to adjust the temp.
If it's taps, take the top off the effected tap, usually there is a screw under the cap on the top, take that off, and get a big adjustable spanner to undo the mechanism,OOPS, I FORGOT,TURN THE WATER OFF FIRST.
Get a scale removing solution together in a container and soak it for an hour, remove it and wash it under running water and try to move the mechanism by putting the top back on it and turning it.
if it doesnt move, you will probably see a spring clip on the bottom of the knurled shaft, remove this and turn the shaft carefully with a pair of sharp pliers, do not slip with the pliers as this will damage the shaft, the mechanism should unscrew down out of the body, you will then be able to get some green pad on the working bits of it and clean it up with the descaling solution to help.
put it back together, (smearing everything with fairy liquid,) then, when you can move it with your fingers, hey presto job done.
The second type of mixer I would leave to me.
It's full of springs and cams, you wouldn't want to touch that.
Derek, June 2004