Please check that the VGA-output/HDMI output of the computer is switched on. For example, on some laptops, you will have to use the Fn+F5 or Fn+F7 to activate the VGA or HDMI output.
Make sure that you are using the original Philips Picopix VGA cable PPA1250, or a certified (mini)HDMI cable
Make sure that you have selected the menu “Source” of your pocket projector
Try by changing the video resolution of the source.