Last night I, too, was unable to fit my rear wheel back into its drop-outs. It turned out that when I had removed the wheel and had completely unscrewed the drive-side nut, dropping the spring coil to the floor, I had replaced the coil backwards. The wide end of the coil was facing toward the cassette, and blocked the wheel from fitting into the drop-out. I'm new at this. Someone pointed out the reversal, put the coil in the right way (narrow end inward), and voila! Back on the road.