Why is my document, including the art board, transparent?

Typically my artboard is white, and the background grey. Suddenly, inexplicably, it's all transparent. My artwork looks normal. Could be I did something but for the life of me I cannot imagine what -- and I cannot find a way (using a View menu, preferences, document setup, artboard settings, etc.) to change it back. I'm guessing it's simple, but I'm stumped.