Hahah I watched that Big Bang episode last night!
Sheldon basically goes crazy, doing all these weird things with fish, and a loom, correct?
Sheldon was "off his rocker" which means that someone has gone insane, crazy or are acting unusual.
praying he "gets back on his rocker" means they wish Sheldon would stop acting so abnormal and return to his old self.
other examples of the expression "off their rocker" would be something like:
Grandma's off her rocker...She bought a helicopter!
-"What is he wearing?!"
-"I don't know, but he must be off his rocker"