I had the same problem. It was actually my first post on this forum, right after I bought my car last Feb. I though I had the wrong key. If you use the key fob and simultaneously lift the trunk real hard, it should open. Otherwise, just spray a crapload of lube/WD40/whatever in there until things get moving. Otherwise, it should open again once it gets warm.

Old car: '98 SVT T-Red K&N air filter; Ford Focus RS shift knob. Off to be auctioned off somewhere. New car: 2003 Honda Accord DX Sedan, 5-speed.