Starter engages but doesn't crank

My 2004 Pontiac sunfire will not start. You can
hear the starter engage the fuel pump is working.
You can hear the crank moving but the car won't
turn over,the top end of the motor is not
functioning. The car ran fine the day before after
getting home turned off the car and wouldn't start
the next day. Never heard anything unusual the day
before. It wasn't running rough or anything . It
never backfired or died on me  it was running great
and sounded fine.   and then next day  would not
start. My boyfriend says You can hear the crank
turning through the exhaust. Was thinking if the car
had jumped time  that it would still start and never
heard  any loud noises my car has a timing chain
not a belt if that had broke wouldn't we have
heard? We have  replaced that battery, starter,
spark plugs  coil, and ignition control module and
still nothing. Was going to try and replace the
crankshaft sensor. Would the crankshaft sensor
cause the motor not to turn over? Is there any
ideas that we could try? Have already put $400 in it
and still won't fire or turn over. Can't afford to just
keep replacing part that doesn't work.