Well, one of your mods is not installed correctly, isn't updated or conflicts with another one.
I'm not sure what kind of help you're expecting if you give no details.
If you have only a few mods installed, just go through them one by one and make sure they are up to date, compatible with game version 2.11 and look if there is any known incompatibilities with your other mods. All of this is stuff you will find on the mods' pages.
Alternatively, you could post your mod list here and maybe someone will immediately recognize a conflict because they saw/experienced it before.
What is certain is that you will need to provide far more information if you want someone to try and help you. Otherwise there is no easy answer here, this is just part of modding.