Missing video clips, Keynote '09 5.1.1 (build 1034) and OS X 10.6.8

I use keynote quite a bit, and have a collection of perhaps fifty keynote talks written over the past five years. But a serious problem has surfaced in the past day: if I open any of my keynote files, the VIDEO CLIPS DO NOT SHOW. I can see the video clip (a static image) in the thumbnail image on the left. I can also see the placeholder if I put the mouse over the video location on the slide. But when I "play" the page, I just see a black box where the video should appear, with the usual play and pause buttons, etc.

If someone has experienced this problem, and/or has a solution, I would be grateful. I have updated my OS on a regular basis, so as far as I know, everything is up-to-date. It may be that a recent update has caused this; the last time that I looked at some keynote files a few weeks ago, everything was working OK.

(NOTE ADDED TEN MINUTES LATER: PROBLEM "SOLVED" BY SHUTTING DOWN ALL APPLICATIONS AND REBOOTING LAPTOP.)

Message was edited by: ProfBA, SOLVED