Steve was arrested at the end of Civil War. He was assassinated by Crossbones (who was a distraction, and shot Cap through the shoulder when cap protected an officer next to him) and Sharon Carter, who had been brainwashed by the Red Skull.
He had been considered dead for quite some time, and was "burried" underwater in Antarctica under the protection of Namor, The Sub-Mariner.
His body was also shown to have reversed in death from his strong physical form to that of a sickly aged man.
Recently, Sharon Carter has discovered that the gun she used to "kill" him wasn't actually a regular gun, but was a device that, in one way or another, trapped him in time. He is currently not back, but is instead reliving all of his past experiences over and over again.
Not entirely sure what is going on right now because that hasn't been really revealed
But hope that helped