djzubek:
I have a sketch written. Can I start the sketch in any way other than applying power to the board?
The answer to your question is "no", but you are asking the wrong question.
If you want your sketch to be doing stuff under some conditions (it's been started), and not doing stuff under other conditions (it's been stopped), then you write a sketch that does that. Starting and stopping is not something external to your sketch, it is part of its functionality.
Perhaps an analogy might help. If you are building a car, it's easy to concentrate on only one part - say, the engine. I mean, a car needs to go, right? But if it also needs to change direction, then you don't manage that by turning off the car, hauling the nose of the car around, and then restarting the car. Going and changing direction are both things that a car needs to do, and if you are going to design a car then you need to design parts that do both of those things. Sure, going may be the most essential part and the part that takes the most effort to accomplish, but the other stuff also needs to be done too.
A model is:
boolean i_am_currently_running;
loop() {
work out if I'm supposed to be started or stopped right now (eg, by looking at a button)
if i_am_currently_running and !I_should_be_running, then
do whatever needs to be done to stop (eg, turn motors off)
i_am_currently_running = false;
else
if !i_am_currently_running and _should_be_running, then
do whever needs to be done to start up
i_am_currently_running = true;
end if
if i_am_currently_running, then
do whatever might be involved in my usual running functions (blink lights, step steppers)
else
do whatever might be involved in my usual stopped function (eg: nothing)
end if
}