Could use a little more information, but I'll take a guess;
If the rear LED blinks green at a rate of 3 per second, a one second pause, and repeats, it means the drone is in Smart Mode and has not achieved GPS lock (or GPS is turned off). It won't start like that. Either put it in Angle Mode, or take it outside to get a good GPS signal.
If that doesn't work, please describe which light is flashing, the pattern of the green flashing, and any beep sounds you hear.