If the cap is stuck, and everything in the release mechanism is OK, then you could have an assistant pull the release while you try to pull it open. If that works, try a little oil on the gasket or the latch or hinges,so they won't stick.
If there's a problem in the mechanism then you need to see a Mazda guy ASAP, before you run out of gas.
PS: There may be an emergency release in the trunk. There usually is. Refer to the owner's manual if you have it for more info.
Here's a directory link for you: