The first two things to check for if your machine all of the sudden quits on you, Contact points, Tattoo Machine Capacitors, Power Supply, Foot Pedal, or Clip Cord. When you check these two, you should also check you other machine to see if it will work. If it in fact is the machine and is not fixable at that moment, you can always switch your needle bar and tube to the other machine and be able to reset the point gap in order to finish the tattoo.
If your machine just won’t work at all, you probably have an open in your circuit or a shortage. You want to make sure all your points are clean at first. The circuit will break if there is something caught between them. To open the contact hold the armature bar down with your thumb and blow hard on the points. If the blowing doesn’t clean them then you can try pulling a small strip of paper through the points when they are closed or use a nail file to clean out the contact. Then if that works you might want to readjust the point setting slightly so that you get that “buzz” at the right sound again.