20" Cinema Display goes dark on 13" MacBook (Lion)

While the user was briefly away from the MacBook, the Cinema Display went dark, though the power light remained on and steady. The display image, which had been destined for the Display, returned to the MacBook's screen, the arrangement-of-displays option in the Display preference panel disappeared, and "Discover Displays" found nothing but the Mac screen. I tried resetting the NVRAM, no change. However, when the machine rebooted, Mail and Safari autostarted on the Mac screen, but iPhoto apparently was off-screen in the universe previously handled by the Display. When I pulled all the Display's cords out of the Mac and restarted, iPhoto came up on the Mac screen. I removed the mini-DVI/DVI adapter from the Display cable and plugged the cable into the DVI port of my MacBookPro -- same behavior. This has seriously impacted the work of the user, who got the DIsplay because of difficulty seeing the MacBook screen. Any advice would be much appreciated.