A number of things tend to cause this, including opening the same proposal in multiple windows, unexpected logoffs from the system, or other users taking control of the proposal while you are working within it.

If you look at the top of the proposal window, you should see the words "Locked by" followed by a username. If that username is your own username, then you can simply click the Lock icon  next to it, and take the lock back from yourself. If that username belongs to another user of the system, you should contact them before taking the lock back.