Make sure that the data projector is on and out of standby mode before you start - they take about 3 minutes to warm up.
Ensure the projector is set to the right input source - Computer or Video.
When you've finished, please put the data projector to sleep to reduce the wear on the (expensive) bulb

Usually the remote is used to power up the projector and select the source:

However, this can also be achieved on the projector itself: