Somehow, your langs.xml has errors in it. Perhaps while exploring various Notepad++ options you accidentally made unintended changes to it. That's what happened to me.
Go to the installation folder for Notepad++ and rename langs.xml to langs.xml.bad. Then, in that same folder, find langs.model.xml, make a copy of it, and rename the copy to langs.xml. DO NOT simply rename the file or you will not have it available the next time you need it.
also replace this file in the folder "C:\users(user)\appdata\roaming\notepad++"
You will want to then compare the langs.xml to langs.xml.bad and see if there is anything legitimate that you want back.
If you are missing the langs.model.xml file, you can download the current version from the svn repo.