Guy was a gym teacher for special needs kids (really), an earthquake left a bus full about to go over a cliff. With no powers guy saved them, but while saving the last one, well, lets just say the bus landed on him. He lived, but was bed ridden (and eventually ended up comatose).

The guardians seeing Hal's backup was no longer fit for service found a new successor, John Stewart. Until a renegade Guardian forced Guy out of his coma to be his personal GL (thus causing Guy's initial 'brain damage')