Double check to see that your level sprites and the player Blueprint they are sitting on the same plane along the Y axis. (Into the screen)
It could be that as you are dragging and dropping sprites into world they are snapping to some object in the world such as the Skybox, or the Player Start was moved off axis.
…or if you happened to start using Tilemaps, be aware that they do not support collision in engine versions prior to 4.8. (You can get around this by creating BSP/geometry behind the tilemap for the player capsule to collide with)