http://www.uscis.gov/files/article/c4_english.pdf

Q: How do I apply for a Replacement Nonimmigrant Arrival-Departure Record, CBP Form I-94 or I-95, if the original was lost, stolen or is mutilated?
A: If you are applying to replace a lost, stolen or mutilated Arrival-Departure Record (CBP Form I-94) or Crewman's Landing Permit (CBP Form I-95), you must file an Application for Replacement/Initial Nonimmigrant Arrival-Departure Record (Form I-102) with the USCIS office having jurisdiction over where you are temporarily located.
If the CBP Form I-94 or I-95 was lost or stolen, submit a copy of the original CBP Form I-94 or I-95. If you do not have a copy of your original CBP Form I-94 or I-95, please submit a copy of the biographic/photo page from your passport and a copy of the passport page that was stamped by CBP officer when you entered the country. If you cannot submit any evidence of your legal admission to the United States, please submit a full explanation and proof of your identity.

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Question
I did not turn in my I-94 when I left the U.S., what should I do?

Answer
If you returned home with your Department of Homeland Security Form I-94 (white) or Form I-94W (green) Departure Record in your passport, it means that your departure was not recorded properly. It is your responsibility to correct this record. You must provide U.S. Customs and Border Protection (CBP) sufficient information so we can record your timely departure from the United States. This will close out your earlier record of arrival to this country.

If you do not validate a timely departure from the United States, or, if you cannot reasonably prove otherwise when you apply for admission to the U.S. in the future, CBP may conclude you remained in the U.S. beyond your authorized stay. If this happens, the next time you apply to enter the U.S. your visa may be subject to cancellation or you may be returned immediately to your foreign point of origin.

In particular, visitors who remain beyond their permitted stay in the United States under the Visa Waiver Program cannot reenter the U.S. in the future without obtaining a visa from a U.S. Consulate. If this occurs and you arrive at a U.S. port-of-entry seeking admission under the Visa Waiver Program without a visa, CBP Officers may order your immediate return to a foreign point of origin.

If you failed to turn in your I-94 Departure Record, please send it, along with any documentation that proves you left the United States to:

DHS - CBP SBU
1084 South Laurel Road
London, KY 40744
Do not mail your Form I-94 Departure Record or supporting information to any U.S. Consulate or Embassy, to any other CBP office in the United States, or to any address other than the one above. Only at this location are we able to make the necessary corrections to CBP records to prevent inconvenience to you in the future.

To validate departure, CBP will consider a variety of information, including but not limited to:

Original boarding passes you used to depart the United States;
Photocopies of entry or departure stamps in your passport indicating entry to another country after you departed the United States (you should copy all passport pages that are not completely blank, and include the biographical page containing your photograph); and
Photocopies of other supporting evidence, such as:

Dated pay slips or vouchers from your employer to indicate you worked in another country after you departed the United States,
Dated bank records showing transactions to indicate you were in another country after you left the United States,
School records showing attendance at a school outside the United States to indicate you were in another country after you left the United States, and
Dated credit card receipts, showing your name, but, the credit card number deleted, for purchases made after you left the United States to indicate you were in another country after leaving the United States.
To assist us in understanding the situation and correct your records quickly, please include an explanation letter in English. Your statement will not be acceptable without supporting evidence such as noted above. You must mail legible copies or original materials where possible. If you send original materials, you should retain a copy. CBP cannot return original materials after processing.