My camera was in damp weather and now the screen has lines on it when you try to review pictures or use the menu on the rear LCD screen. Can this be repaired?
You may have a buildup of moisture inside your camera causing the screen to short out. you could try packing your camera with a desiccant like silica gel packs (those little "do not eat" packs you find in shoes), some people have had success with instant rice in a pinch (do not bury your camera in rice). this will help absorb any moisture and may correct your problem.
Otherwise you may have to take you camera in for repair. Contact Nikon to find certified repair shop.
If @Evan Oswald's answer (silica gels or rice to dry it out without opening) does not help, it might be enough to open it up, and let it dry that way. You can also replace some parts.
You lose any warranty, if you still have any. Also Nikon won't repair it for your afterwards even for money. I'd decide whether the camera is worthless to you right now. If you decide it's worthless you might as well try to fix it. iFixIt has a repair guide for the LCD on the D90.
Trying to completely dry the camera certainly won't hurt. But the fact that the problem hasn't already gone away doesn't bode well.
I guess the big question is if it's the camera's electronics or the LCD itself. Inspect the screen very closely to see if there are any tiny cracks- moisture could have gotten in but not been able to get out. If you see any, use silica gel until it works and then try putting some clear nail polish on the crack(s). I've personally done this not on a camera, but on a rowing stroke meter/speedometer that I um, kind of stepped on :(
A cheap source of silica gel is flower drying powder from an arts and craft store. One package will last a lifetime because you can pop in in the oven to refresh it. It comes in bulk, so you'll need something to hold it- I've used socks.