Sounds like you have a document with Protection applied to one or more sections, and you just entered a section where editing is disabled.
Have a look on the Tools menu for the command "Unprotect Document". If it is there, your document is locked. If it says "Protect Document", that's not it.
In which case, I need a lot more detail about what you're seeing. Remember, I can't see your screen from here, so you to tell me what you are seeing.
