Man, you should have gone back to the custom office and filled out another one right there. Now you have to gather alot of paperworks to prove when you landed here. To start the process, you can go to your local USCIS office and tell them that you lost your I-94. Of course, they will give you a form to fill out. Fill it out and submit it with copy of your ticket, passport, visa,  I-20, previous I-94.