Engine won't start. Makes a loud grinding or whining sound instead

My 04 Ford Explorer Sport Trac's engine won't turn over. I
went to go and pick up my daughter from school the other
day and the truck started and drove fine. I had to get out to
go get her and when we got in to leave the engine didn't
start and have a loud grinding/whine sound. I had to have it
towed to my house because the mechanics shops were
closed by the time the tow truck got to me. Any ideas on
where to start looking would be greatly appreciated. I've
had people tell me it might be a starter problem, or a
sensor somewhere so I'm just looking for clarification on
where to start to try and diagnose the issue.