Originally posted by Dennis_300E
Just another obvious one. You're trying to open the gas door with the car unlocked, right? Locking the car also locks the gas door.
Make sure the car is unlocked, and then push on the right side of the filler door to open it.
If that does not work, then there is a override release in the trunk close to the filler neck under the carpet. If you have trouble finding it, look in your owner's manual, or swing by a dealer, and they'll show you.
If the filler door is not unlocking when the doors and trunk are unlocked, than you'll have the task of diagnosing that problem, and should preface your post with "vacuum problems" as there are some knowledgable folks here that will be able to help with vacuum lock problems.
Also, do a search for "vacuum door lock problems" and see what pops up.