If this is the router with the plastic ring that you spin to adjust depth, I have suggested a solution to someone once before that worked. My suggestion was that he heat the ring gently with a heat gun and expand it enough to get it moving. He didn't have a heat gun but used a hair dryer on high instead and it worked.
Once you get it moving you need to clean the threaded part of the housing with a wire brush to remove the scale and corrosion that has built up underneath.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.