Take a look
here on various methods to degauss TVs.
Your set should have an automatic degausser, it just only works from a cold start and may require many cold turn on cycles. I gave my son a new TV for Christmas, and on the day after Christmas he moved it up to his room. Somewhere along the way the screen got gaussed (?!), and stayed that way until yesterday, when the last of it finally cleared up. It must have taken a dozen or more cold starts (TV off for 30 or more minutes) to do it, but it worked. Patience is the key.
Good luck!