Have a look here and identify the model :
http://www.icomuk.co.uk
I suspect that the squelch pot has seized and you have stripped the inside of the knob. Seems to be a common problem.
Try pulling the knob off and then seeing if the pot underneath moves. If it doesn't then apply WD40, clean up the pot and gently try and get it moving with pliers. Then contact the nice Mr Jon ICOM Brooks for a new knob.