Bronco460fi

Hi guys I have a 20 defender xmr. It has about 1700 miles on it had it out this weekend and it started making a noise that I thought was gears not meshing correctly then stalled. Wouldn’t start back up seemed to have a low battery so I charged it and grit to start again after work today. Still the same so I decided to drain the oil and it was full of coolant. My heart sunk. Anyone have any idea what could have caused this? And what are the odds of a complete rebuild? Thanks for your help!