If you were ordered removed (or deported) from the U.S., you cannot simply turn around and come back. By the terms of your removal, you will be expected to remain outside of the country for a set number of years: usually either five, ten, or 20. It's even possible that you will not be allowed to return to the U.S. at all. It all depends on the reason you were deported, as described below.
First, it's important to know whether or not an Immigration Judge (IJ) actually issued an order of removal against you: particularly if you were ever told to go to immigration court but you don't know or can't remember whether you went or what happened there. Some of the possibilities include that the IJ:
If you were granted voluntary departure but failed to leave by the appointed date, the grant automatically converted to an order of removal. Also realize that even if you voluntarily departed the U.S., you could still be inadmissible for a number of years, depending on how much unlawful presence you accrued while in the United States. To learn more, see Consequences of Unlawful Presence in the U.S.—Three and Ten Year Time Bars.
To check whether the IJ issued an order of removal, call the Immigration Court customer service line at 1-800-898-7180 (if you're in the U.S.) and follow the electronic prompts.
If you do, in fact, have a deportation or removal order in your immigration file, it's possible that you aren't allowed to apply to enter the U.S. for five, ten, or 20 years. The applicable law comes from Section 212 of the Immigration and Nationality Act (I.N.A.). Here's a quick summary:
If you were summarily removed or deported upon arrival at a U.S. port of entry because you were found inadmissible, or if you came to the U.S. but were immediately put into removal proceedings and then removed or deported, you might be ineligible to return to the U.S. for five years. The five-year ban also applies if you failed to show up for your removal hearing in the United States.
If an IJ issued a removal order at the conclusion of your removal hearing in Immigration Court, you may not return for ten years after your removal or departure.
If you were convicted of an aggravated felony or have received more than one order of removal, you are barred from returning to the U.S. for 20 years. And if you entered the U.S. without permission after having been removed, or illegally reentered the U.S. after having previously been in the U.S. unlawfully for more than one year, you are likely barred from entering the United States for 20 years or permanently (in that you'll need special consent to return).
If you were ordered removed from the U.S., there are limited options that might enable you to return legally.
First, you'll need a basis for applying for a U.S. visa or green card. Sponsorship by an employer or a family member are possibilities, for example. You might also qualify for a nonimmigrant visa, such as a tourist visa or a student visa.
Before you can apply for any of these, however, you would need to apply for a waiver that could "forgive" your prior removal order and give you permission to reenter the U.S. with the visa or green card.
To learn more about applying for permission to enter the U.S. after deportation, see After Removal: Possibilities for Reentry to the U.S.
If you have a 20-year ban against you, however, you must wait until you have been outside the U.S. for at least ten years before so much as applying for a waiver.
Returning to the U.S. after removal is a complicated and difficult process. It's best to seek legal assistance as soon as possible, to discuss your case and the options available to you. Many attorneys can work with you remotely even if you are not physically present in the United States.