There is a procedure for obtaining a replacement I-94 card - USCIS has a form you can file to request a replacement when one is lost or stolen.
However, you would need to be able to present some proof of exactly when and where you entered the U.S. Without the pasport, the best proof is really boardingt passes from the plane stating date and time of arrival as well as place/airport (I know, almost nobody ever keeps these).
You might try checking with the airline, travel agent, or someone who travelled in with you who might still have the passport/I-94. If you can find some sort of evidence, these aren't hard cases - finding the evidence is the tough part.