His hammer is just a catalyst
After Odin took his power away, the hammer became a sort of key to unlocking it.
In the comics, he never really gets his full power back, and during the Thor: Unworthy storyline he forges a new hammer to unlock his power again.
Part of the crushing effect losing his hammer must have on him is the partial loss of his power, which is why I'm sure this reveal with him embroiled in lightning is so important, and why Hela destroying it such a pivotal plot moment.