Help please...

Can anyone help me with this issue, only get a clicking when trying to start. Has new starter, good battery, even with a high amp jump box starter, or a car still just clicks. Was running and replaced alternator belt, it squealed a little-tapped gas and belt split lengthwise in half. Shut car off immediately and will not start since this happened. Has new belt, oil, filter, plugs, even put thicker ground wire for starter. It will engage the starter to the flywheel but won't turn motor over. And motor is not locked. PLEASE somebody know what the heck is going on!!