It's been an age since this was posted but thought I'd assist someone in the future!
Just had this issue on one of mine and started to panic as the backup was on a CF and never tested.
Before I sent out needed to pull a backup but couldn't as the screen was white...
Thought i'd remove the back cover and have a looky see. Disconnected one of the back light cables (top one) and tried it again. Bingo all good. It must have failed to full brightness so it was all there but couldn't be seen.
Since performed a backup!
Hope it helps someone in the future....