+2 on David's reply. Assuming this was an extract batch, and you added some top-off water, the most likely cause of an apparent low gravity is that you didn't mix the top-off water with the wort thoroughly enough. This is nothing to worry about as the yeasts will do the mixing for you. If you were using extract, then your real OG will be very close to what it should be. If you say what the recipe was, and what volume you made, we can check what the OG should have been.
-a.