Hi,
I have a design that will take me 10 hours of run time to accomplish. I haven't experimented yet with the commands on running a g-code.
My question is; How would I go about stopping the g-code from running and come back to it the next day? Can this be done, do I need to leave everything on (Mach3, computer, motors etc.) or close everything and start from a certain line of code? Hope I'm making myself clear
Any comments would be appreciated 

Here is what I do. Press stop (red button on left lower corner) on the screen .... I turn off the router & leave the computer on. Next day turn on the router & press start (green button on left bottom corner) and it starts where you left off. I have a laptop but it has no sleep on it. If you have sleep it may not work.

Do not press Stop to stop the machine. It will lose position.

To stop without losing position, you need to use Feedhold, then wait until the machine stops, then hit stop.

Also, turning off a stepper powered machine and then turning it back on will usually result in a loss of position of a few microsteps. This may be close enough, though.

If you don't have home switches, and a repeatable home position, then the best thing might be just to use feedhold, and turn off the spindle. Then turn the spindle back on and use Cycle Start the next day to resume. But you'd need to leave everything powered up.

If you have home switches, then you can stop the machine (feedhold first) and then turn it off. Make sure you write down the XYZ offsets to make sure they are the same the next day.
The next day, you turn on the machine, home it, make sure the offsets are correct, and use run from here to resume.