Are you referring to your gas tank cap? Is it held on by any kind of tether? (my civic has one so I don't leave the cap on the roof or somewhere) If you don't have a tether, it's just a matter of buying a new cap and screwing it on until it clicks like you would when you're done filling up with gas.
Did you fail an emissions test because the gas cap wasn't sealing? Is it possible your Rainier is still under the emissions warranty? hope this helps