It depends what method of birth control and how long you were on it.
With pills, your fertility can resume immediately but also can take several months. Just as it takes several months for your body to adjust to being on the hormones, it can take several months for your body to readjust being off of them.
With the shot, it can take several weeks to several months to regain fertility, and it can take several months for your body to re-adjust.
Stress (like worrying over if you're pregnant or when your period will come, or unrelated like financial stress) can also delay your period.
If you are concerned about it, call your doctor and discuss your concerns. You may also want to get a checkup to make sure there is no medical reason for the delay.