My 3-week-old PRS-350 has a broken screen and I don't understand how this happened.

I read it in the evening, then left it on the table. Nothing unusual happened. The next morning, the screen showed dark areas and wasn't working anymore.
I took good care of the reader (and it was always in a Sony cover), so I'm really surprised. I had a PRS-500 for years before this and never had problems. The store where I bought the PRS-350 refused an exchange because they think I broke the screen. SIGH.
There is nothing I can do about this, but I'm wondering if anything similar had happened to other people here as well. I love the PRS-350 and I'll have to replace it, but this experience is rather disconcerting.