Every BYD, electric or plug-in hybrid, has two electrical systems. The big traction battery drives the car. A small 12V battery runs the locks, lights, screens and the computers that switch the high-voltage system on. If the 12V side is flat, the car stays dead even with a full main battery. On many BYD models that 12V battery is a lithium iron phosphate unit. On others, depending on year and market, it is a conventional lead-acid battery.
Here is what can be done where the car is parked. The 12V battery can be tested, and a flat one can be boosted through the points shown in the handbook so the car wakes up and recharges it. A lead-acid 12V battery can be replaced on the spot. A lithium 12V unit is a model-specific part with its own electronics, normally supplied through the dealer. No mobile technician should open or work on the orange high-voltage cables.