N3 Notes locked, grey line

As I entered notes for the trumpet part, I quickly typed keys on the LHS of the keyboard. I suddenly found all notes (for that part only) locked. There are thin grey horizontal bars running through all notes (but not rests). The notes can be cleared and can be moved w/ up/down arrow keys, but they cannot be moved with the mouse.
Once done, I could not "undo" it.
What have I done? I cannot find any info in the User's Guide.
Thanks!
Once done, I could not "undo" it.
What have I done? I cannot find any info in the User's Guide.
Thanks!