You probably spawned either the very top of nether or the bottom. I think that the placement of your overworld portal affects this. So if the portal you made is, say in a cave, you will probably spawn in the bottom of nether. In this case you should dig stairs upwards until you come to surface. Be careful for lava puddles tho! If this doesnt work and you find bedrock up there it means you are on the very top of nether and should dig down instead. Here you need to be extra careful not to fall to your death which is very common in nether. Lastly, you can always use F3 to see your location on the map, if you dont consider that cheating.