If your Green Card expired, and you are in the United States:
You do not need to worry. You are still a resident. You will not lose your condition.
You just have to request its renewal online through the I-90 Form six months before it expires. If the Green Card expires in less than six months to the date or if it has already expired, it will not be possible to apply for the citizenship by naturalization.
If you did not fill in the I-90 form before leaving, you will probably have difficulties, because this form cannot be completed from abroad. Besides, the airlines will not let you fly to the United States if your Green Card has expired. Therefore, it is very important to take precautions.
What Can I Do?
You have to set an appointment at the nearest U.S. consulate to request a Transportation Letter. You will need additional documentation, such as 2 passport photos, a legible photocopy of your passport, a copy of the boarding pass issued by the airline and the expired Green Card. It may be necessary to file a police record.
When you receive the Transportation Letter, you have to travel to the United States before the due date, and not 30 days after it was issued. It is very important not to confuse the permanent residence with the conditional one.