Help us help you -- How about a little more information:
iPhone or Android? (model and operating system)
Have you updated the phone app?
Can you not see the camera view?
Do you see the battery life, satellites, etc?
Did it work before?
Can you take pictures or videos using your phone?
Can you tilt the camera using your phone?
Suggestions:
Can you try it from another device (borrow a friend's phone?)
If you see everything EXCEPT the picture, maybe try setting the FPS on your device to the lowest one and restarting?
Save all your data and try reformatting your SD card (use the DJI app to reformat!)
Try a different SD card
Double check the little camera cable -- unplug it and plug it back in
Double check the camera switch!
And lastly, did you happen to leave the lens cover on? (not trying to insult you but I did that once and I'm embarrassed to say how long it took me to figure that out!)
Good luck!
tcg
