What is litre size of engine? Your saying the starter motor won't crank?
Turn on the headlamps, turn key to crank, Is there any reaction from headlamps? If the lamps go out or almost then usually, suspect the battery or cables. If the lamps stay bright, maybe an open circuit to starter? If this is an automatic tranny, could be an issue with neutral switch. Could be an issue with starter itself? Some of the national brand autostores will test the starter for you.
Have you done any voltage testing at the starter, itself? Battery voltage at the starter is hot all the time. Crank voltage at the starter goes hot with key in the crank position. Any testing at the starter, make sure tranny is in park or neutral and the parking brake is set.