Your monitor may be off for a number of reasons. Under Settings > Display, if you have selected the Backlight to be off or the Backlight timer setting to be on, then the screen will be blank when you are not using it (or it is scheduled to be off). Touch any button and the screen should come back on.
If the monitor is being powered by batteries, then it may be that they have run flat. Either connect your monitor using the supplied power adapter or replace the batteries. The monitor will run for approximately 4 hours while on batteries so that you can carry it around the home to learn how different appliances work. It is not intended for prolonged use.