Hi,
Are your periods typically on time, or are they irregular? If your periods are usually on time, you should take a pregnancy test. Keep in mind, that irregular periods aren't uncommon for young women. Since it's been nearly two years since your last Depo injection, your menstrual cycle should be back to a regular schedule by now. If you continue to miss periods, you could always speak with your gynecologist about your concerns.
Good Luck