Tracker,this happened to me a couple of weeks ago,I did mention it on a thread,I phoned the company from whom I purchased it,and they said to remove the battery,then replace it again,straight away,and it was fine.I may add that it was a very cold day,and it was the first time it had happened,I have been using the camera since the late Spring.That was the reason I purchased a used 990 as a back up.
Malky,I went to the Nikon site to do as you suggested,but could make no headway at all,far too complicated.I got no further than page 1!!