Do you mean your scared of not being able to get pregnant after you STOP using it? They say not to use it for more than 5 years. Also when you stop taking it, it could take up to 9 months to ovulate properly. That is only with long time usage though. If you only used it once, it shouldn't affect anything at all.
By the way, after I stopped using depo, I ended up getting pregnant. I used it for over 5 years. Sadly my pregnancy ended up in a m/c (unrelated to depo, it's been about four years.. hubby was using the pull out method until we were ready to try) and now I'm ttc again.