If you have applied to immigrate to the United States, most likely under either the family-based green card category or the employment-based green card category, you might find that you have to wait several weeks or months to hear back from the office processing your case. Most likely, that is an office of U.S. Citizenship and Immigration Services (USCIS).
Don't panic! Such delays are normal, although certainly bothersome. At least USCIS offers various procedures by which you can check the status of your petition or application.
After you submit a petition or application to USCIS, its practice is to send a receipt, usually within several days or weeks. Look on your receipt notice for your 13-digit receipt number. The number will begin with three letters to identify the office handling the application, such as EAC, LIN, SRC, or WAC. Your receipt number will be helpful in tracing your case through the system.
If you do not receive this number after several weeks, call or email the USCIS Contact Center. It might be able to resolve matters over the phone. Or, it might make an appointment for you to visit a USCIS office.
In the latter case, you'll want to bring personal identification, copies of your application, and any other materials relating to your case. If you sent the application by a courier service or certified mail, bring the receipt, so that you can show the officer that you really sent it.
To check your immigration status online, go to the USCIS "Case Status Online" page and enter your receipt number. It's a good idea to also sign up for case updates from USCIS by creating an account (on the same page).
Another important area of the USCIS website is the Check Case Processing Times page. There, you can find out how long the various USCIS offices or service centers are currently taking to process different types of applications and deduce whether your application has been lost in the shuffle.
If you don't have access to a computer, or have discovered that your case is taking longer than anyone else's, you can contact USCIS as described above.
Another option, if the last you know of your case is that you sent it to a USCIS lockbox (an initial processing service center, which is supposed to forward applications and petitions to USCIS service centers) is to email the lockbox using one of the address provided on the USCIS "Contact Us" page.
Keep your emails to the U.S. government polite, making sure to include all relevant facts such as the petitioner and beneficiaries' names, the date of filing, whether your filing fee check has been cashed by USCIS, and so forth.
If you're still not getting anywhere, consult an immigration attorney for help. An experienced U.S. immigration attorney can help you figure out what is happening with your case and will know what sorts of delays in processing are to be expected. In some cases, the attorneys have ways of reaching someone within USCIS that ordinary people don't.