The serpentine belt idler pulley fell off my chevy 2001 silverado truck. How do I get it back on and why did this happen?

there are 15 serpentine diagrams for the belt on vw engines to 2001
some have no ac , power steering etc
need engine size
you could try this the grooves are for driving pulleys so any grooved pulley it has to go around
flat pulleys run on the back of the belt so they will be idler pulleys
start at the crank and work to the left if there is a flat pulley between two grooved pulleys the back goes around it and so on

Those belts have a tensioner on them. IF the belt is too big then the tensioner might not be retracted. It might be missing. There are idler pulleys too. If you have high miles on the this car, the pulley or the tensioner might be worn. The tensioner also has to be pulled back to have the belt put on.

with the engine off, grab each pulley that the belt rides on. You are looking for one that is loose. When you find the problem, replace it and install a new belt.

REPLACE THE TENSIONER IDLER PULLEY.THE TENSIONER IDLER PULLEY MAINTAINS THE BELT TENSION WHEN IT GOES BAD.THAT WILL LEAD TO LOOSE BELT TENSION CAUSING BELT TO BREAK.

1=crank
2=alternator
3=idler tensioner
4=IDLER
5=fan idler
6=air cond compressor
7=accessory drive belt(serpentine belt)
8=power steering pump

just in case this is the timming belt and pulleys

GM use a particular item on one of the idlers
there will be a 1/2" square hole in the center of it
use a strong bar and lift the pulley up against a spring
install the new belt and when properly installed let the pulley back onto the belt

The belt has tension applied to it from an idler pulley arm. you need to release tension by using a socket and ratchet on the nut on the idler pulley. Release tesion and slide belt off . Make sure you have the diagram for serpentine belt. it is usualy under the hood. If not make a diagram yorself before removing belt.

its right above the crankshaft pulley,it is smooth and you will see the ribs of the belt because the smooth side of belt is on this smooth pulley

There is a pulley out of alignment. Only 1/8th inch out will cause a serpentine to sqeak.

remove the belt to ac compressor.take your serpentine belt go around crank pulley goes around power steering pump goes around alternator and the tensioner on the far left. the water pump on the out side and the top idler pulley out side. the belt drives them from the out side. then you put your ac compressor belt back on.