I believe a new i-94 is issued each time when you enter the US (likely in exchange for your old i-94). I also have unused i-94 cards that came with i-797s (approval notices for my H visas) from years back. "Unused" because I never left the US while on H visas.
So check your passport and also i-797 notices. Good luck!
Also, if you cannot find it, it's not the end of world. I heard there is a way to get copies of i-94s from the Dept of State or something. Sorry I'm not familiar with how, but you can search the forum on your own..
