U4 not showing anything? :(

We received the projector a couple of days ago and were impressed with the image size + quality.

Today, we were watching a movie, paused to answer the door, then turned back and the projector was on (by virtue of the power LED and fan operating) but no picture.

We tried connecting/reconnecting the HDMI, disconnecting power supply and leaving for a few seconds, nothing.

Any ideas?