On occasions, when I leave my Pi unattended for some time, I can't wake it up by the keyboard. The screen has blanked as isual but the keyboard is unresponsive.
The Pi is left at the command prompt, is not running any apps, connected to a stable 2A psu, powered USB hub with keyboard and mouse plus wifi dongle. The os is the official download with update and upgrade ran.
The keyboard and mouse LEDs are off so it looks like they 'lost' the Pi. Oh and the Pi seems to be running as its led is on and the board is warm. The Pi does not crash or fall over during use.