if the battery is fully drained, you need to give it a chock therapy. Open the iphone, connect the +/- of the battery directly to a power supply with higher voltage than the battery nominal voltage (2-10V, 20-50% of nominal voltage) for 10seconds -1 minute depending on the voltage (the higher the voltage, the shorter the time). Then charge it normally. If the normal charging still does not start, repeat with increased voltage or increased time (works 100%).