Check for fuel and spark to the cylinders
Make sure connections are on tight.
Check your starter, and all power connections.
Check your wire from battery to starter, sounds like it may have been pulled loose.
Did you check your starter?
the bronco needs to have the ground wire connected from the battery to the body. if you replaced the battery and disconnected that ground it simply won't start.
It could be the starter in it. It may be weak or need replaced.
It could be an alternator. Check with a mechanic if that is the issue.
Voltage Regulator?
altinater is probobly dead
check your battery, maybe it's time to replace it
Your belt is correctly tensioned?
Try replacing the spark plugs /wires and distrubuter cap if it doessnt work still you may have a bad ignition coil or fuel problems