[walker]

Sounds like your 12v wire to your coil has a bad connection. You have a 12v volt wire from the starter to the coil to provide power during starting and one from inside the cab, that is a resistance wire, that powers the coil during running. Check out your coil to see if it has two wires on the + side. Chase them back till you find the one that goes to the cab. follow it inside to make sure it is not disconnected from the fuse box. Also, check to make sure there is not a loose wire on the back of the ignition switch.
As a temporary measure run a jumper wire from the battery to the + side of the coil and start the motor.