Your rear derailleur is not broken, your hanger is bent: a 5-minute check

Shifting got crunchy after you laid the bike down? Before touching the barrel adjuster, sight down the cassette from behind. The derailleur cage should be perfectly parallel to the cogs. If it tilts in toward the spokes, the hanger is bent.

A hanger alignment tool is $30 and pays for itself the first time. In a pinch, an adjustable wrench on the hanger (not the derailleur) and gentle pressure gets you home.

Aluminum hangers are designed to bend so your frame does not. Most are $15 to replace. Keep a spare in your bag.