It sounds to me like you're trying to open your trunk with the valet
key. This is identical to a standard key but has two small cutouts at
the tip. It's intentional to prevent a parking attendent from stealing
out of the trunk. To complete the treatment, there's a physical toggle
switch on the trunk latch assembly that prevents the lever by the
driver's seat from opening the trunk, too.

When I bought my 2000 Sonata a few months ago, the seller only gave me
one key -- the valet key. When I thought about it, I realized that with
only this key, it was possible for me to lock myself out of the trunk if
I flipped that toggle. In that event, the only way back into the trunk
would be either with a blowtorch, or by ripping apart a rear seat back.

I quickly got myself to a Hyundai dealer who made me an original key on
a computerized key cutter, from a digital pattern. At $12 each, I bought
two of them.

A later discussion with a locksmith revealed that the crafty dude is
able to make a "real" key from the valet key -- the locksmith can
ascertain how to modify the profile in order to create a standard
door/ignition key.

Richard