The rear side automatic window is not working. The glass fell into the window well. It might be the motor.
From the information provided on your Chevy Avalanche, I can assume from my experience that it is likely that the window regulator has failed. A failing window regulator would result in the window falling inside the door. In most cases when the regulator is replaced a new window motor comes with it as well, that way it will cover that possible issue as well. This is when I believe it is best to enlist help the assistance for a certified mechanic. They will be able to remove the door panel and inspect the fallen window and perform the necessary repairs quickly and accurately.
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