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That dial, if frozen up, is probably corroded in one of its metallic linkages, by splashes of sea water (at a guess) or sea spray; such salty or brackish water or spray is very corrosive.
It may be the dial had a seal protecting the spindle/workings, that broke, letting corrosive saline or brackish water in, also.
The whole dial's basis will have to be replaced, at minimum, I suspect. Especially if the problem has gone on for some time... if it just happened recently and not when tried after a long interval, then it might perhaps be un-stuck by applying some penetrating oil. But, if you try this, do it with the works of the device open, and be careful not to get such oil anywhere else, where it could damage other components considerably over time.
Or you could simply have a Raymarine repair shop replace the dial's potentiometer (or whatever part is causing the dial to not turn.)
How old is the Raymarine device in question, also?