Why crankshaft pulley come loose ?

Look at the pulleys with the belt off. Are they aligned? If so then one of the pulleys is bad. This would cause a belt to come off tract. If the balancer is out, check the bolt, tighten it back to where the balancer is back in line with the rest.

CHECK SEE IF CRANK SHAFT PULLEY BOLTS LOOSE.IF CRANKSHAFT PULLEY LONG BOLT TORQUE IS FOR 130 FT LBS FOR 1.8L ENGINE. IF YOU HAVE THESE ENGINES CRANKSHAFT BOLT TORQUE IS 174 - 181 FT LBS ENGINES ARE 3.0L 3.2L 3.5L 2.3L 2.5L IF CRANKSHAFT PULLEY BOLT IS TIGHT.THE CRANKSHAFT PULLEY ITSELF BROKEN NEED REPLACING.

You have a clutch fan on this vehicle. you need to make sure if it is the fan that is loose at the front of the pulley or the pulley is loose at the water pump. If it loose at the pump then replace the pump. And all of the pullies should turn, with the exception of the bottom center. this is the crankshaft pulley that drives the other ones. it will turn if you put a wrench on it

If it's the main pulley that connects to the crankshaft and drives the serpentine belt, it's called the dampener pulley.
If the dampener pulley has worked loose or separated, you'll need to replace any damaged components, possibly including the pulley, mounting bolt and even the crankshaft and front engine housing could have been damaged if the pulley continued to turn while it was loose.
It could be as simple as tightening the bolt or as severe as a major engine rebuild. A mechanic can't know until he takes a look at it.

TURN CRANKSHAFT BOLT COUNTERCLOCKWISE TO LOOSEN. YOU HAVE RIGHT HAND THREADS.ENGINE TURNING CLOCKWISE.YOU NEED A STRAP WRENCH HOLD CRANKSHAFT PULLEY WHILE LOOSEN IT.BUT YOU NEED A CRANKSHAFT DAMPER REMOVAL TOOL.OR HARMONIC BALANCER REMOVAL TOOL.YOU NEED STRAP WRENCH TO HOLD CRANKSHAFT PULLEY WHEN TIGHTEN IT.TORQUE CRANKSHAFT PULLEY BOLT 90 - 98 FT LBS. MAKE JOB EASIER.HAVE A HELPER HOLD STRAP WRENCH WHEN YOU LOOSEN OR TIGHTEN CRANKSHAFT PULLEY.

when the crank pulley came off it may have hit the other pulleys or the belts could have damage the timing cover. have your mechanic to check the clearance of all belts

Many have been fixed by welding the pulley back on.
It's better than changing the complete engine!

just a socket set will do the job.But remove the bolt and put thread lock-tight on the threads and tighten to manufacturers toque

the pulley is also a dampener often times they seperate from each other. replace pulley/dampener.

Hi your alternator pulley is hold by a nut about twenty mm, chech it it may be lose......good luck