Thanks, Jim. Problem solved. It was scary though. I first went through the whole search for Settings on the phone, with a totally black screen, as if I were blind... Then, using VoiceOver in total darkness, I was able to tap my way to deactivate VoiceOver - and the screen lit up immediately again.
I later discovered that, in VoiveOver mode, You turn the screen off when you "Triple-tap with three fingers." So that's what I must have done inadvertently at some point. What I didn't know was "Triple-tap again with three fingers to turn on the display again."
Thanks again. - Laurent