1. Please replug and check again the ribbon cable connection to make sure the cable is not reversely connected.
2. If the connector between the two layers of PCB boards comes loose, the camera will not power on with no lights on. Please separate the two layers of PCB boards and reconnect them carefully. And make sure below screws are screwed tightly.
3. Please remove the ribbon cable and power up the PCB separately to see if it can be turned up.
4. Please power your Split with a phone charger(5V 1-3A) via a USB cable. If it turns on, please check if your BEC/VTx/FC 5V supplies enough output current(Split Mini's working current is about 650mA under 5V input).
5. Please show us a video illustrating the situation and make sure the status of the LEDs are shown. And then disassemble the camera and take off the PCB and show us HD pictures of both sides of the two boards.