Since it's an 2004 I assume its a coupe and before I contacted a locksmith I'd try to go through the rear license plate light to get the rear hatch open. If you are able to do that and it works then do as Zeracer stated and replace the battery and resync. Good idea to order a second key if you don't already have one. Assuming after that you are good to go then contact a "qualified" locksmith who can pull the lock cylinder from the door and figure out why the key can't be inserted all the way. Good luck.