Was the engine cold when it wouldn't shift? If it's like my 300 SE it will hold 2nd gear for a minute or two until the engine reaches about 50 degrees C, or if you rev it past 3000 rpm, it will go ahead and shift. This is normal, designed to help pollution and fas warm-up. Maybe when you backed up, the fuel moved away from the pump pick-up and you just ran out of gas for a moment???