I have met this phrase watching The Big Bang Theory TV-series. The full scene is: Sheldon took offence at Leonard and got freaked out (as it was called in the movie) because he has called Sheldon's mother. All characters except Sheldon prepared to eat, and mother is praying before start to lunch:
Oh, Lord, we thank you for this meal and for all your bounty. And we pray that you help Sheldon get back on his rocker.
No one literal meaning of rocker I have found doesn't seem to fit the situation. Though I have found an idiom off your rocker. So is get back on his rocker means here something like that he needs to get back to a normal state?