WordPress site went offline after clearing server cache

Without knowing more let me take a stab at this . . .

Have you tried reloading the instance? Yes I'm seriously asking.

A 521 error indicated a server side error. When you dropped the cache the CPU load probably went way up. In the future, I would be hesitant to clear the cache if I am trying to save memory. Linux tries to use all of the memory and the cache will use as much as it can, but this is low priority usage of memory. The cache gets swapped out if another more important process needs the RAM.

If this is a busy site you may want to snapshot the image and resize to a larger instance type.