Happy 2010.

Try using the trunk key.
Make sure to turn the key before pushing the
trunk button in.
I think the key and slot should be in a Vertical position
in order for the trunk button to unlatch the lock.

Good Luck,
Joseph in New Mexico

You can always get about a 4 foot extension with a 10mm socket on it and remove the 3 bolts that hold the latch on and then open the trunk. I had to do that with a 242 I recently bought that didn't come with a set of keys. Good Luck!

As others suggested: this is ice (usual problem) or wrong key.
--
240s: 2 drivers and some parts cars

If the trunk is locked using the key and then removed leaving the key slot in a horizontal position the central locking solenoid will be unable to unlock it.

Is there a chance that is what has happened?

It is also possible to open the trunk by reaching in through the ski hole by manipulating the mechanism as shown here:

Maybe it's frozen? After a wet snowdump last week, a brief thaw, then a hard freeze, mine was effectively welded shut via ice on the weatherstripping.

Make sure you have the correct key.

What you are describing is exactly what a parking lot valet might expect to find when he decides to add your jumper cables to his collection.
--
Art Benstein near Baltimore

If you tell the truth, you don't have to remember anything.