I've read your mind from this vague post and the solution is to reinstall your game.
On a serious note, being vague won't get you any useful help since people have to guess what the hell you're talking about.
Please provide details about what you did/installed/etc. so that we have a better spot to start from in order to help you. what mod(s) do you have installed? what mod(s) did you install before this occurred? Did you follow the install instructions from the mod description or readme file?