The Particle accelerator Explosion caused Ronnie and professor stein to merge to form the FireStorm.And It made everyone think that they are dead but what they didn't know is that the dark matter released from the Particle accelerator Explosion hit them and made them aka The FireStorm.For some days star labs didn't know that they are alive and Ronnie's girlfriend also thought he was dead but he was alive in the pipeline with professor stein merged in him.
And professor stein took over Ronnie's body but Ronnie was somewhere merged down in his body while professor stein took over his body. One day finally Ronnie's girlfriend aka killer frost found him alive in the pipeline and called her best friend cisco aka the vibe and took him into the pipeline.