According to [link|https://docs.px4.io/main/en/getting_started/led_meanings.html|this troubleshooting document|new_window=true]
* ***[Solid Purple] Failsafe Mode:*** This mode will activate whenever vehicle encounters an issue during flight, such as losing manual control, a critically low battery, or an internal error. During failsafe mode, vehicle will attempt to return to its takeoff location, or may simply descend where it currently is.
Did you [link|https://docs.px4.io/main/en/flying/pre_flight_checks.html|check for preflight errors|new_window=true]?