The first thing you need to do is to ascertain if the battery has any charge in it at all - without this, nothing will happen. However, many machines have things called interlock switches fitted to them, which prevent starting if there is a hazard. For example, my own tractor won't start unless you are actually sitting on the seat, due to its seat interlock...until I disconnected it, of course! Cheers John....emerging from a long English winter...!
John, April 2009