Being a Mitsubishi you have to expect this kind of trouble.

Get a small jimmy bar and place it on the lip of the fuel filler flap and
pry it using the body as the fulcrum.  You will probably damage the body
slightly doing this, but at least you will be able to put fuel into it.

Have someone pull on lever while someone else is tapping and or  pushing  on
fuel door. As long as lever is pulling cable it just needs to spring back.
After you get it open, spray some wd-40 on spring plunger inside fuel door.
Don't use prybar.  You can also use a small slim jim to slide around fuel
door to unlock  if everything else fails.   Later,  GSXAWD@kc.rr.com

Is it not releasing or is it not springing open after release? On my
daughter's '00 Mirage there is a little spring thing that disappeared.
Replacing it was the cure. Got a replacement at the dealer.

There is usually a secondary release method. On many cars there is a
release handle in the trunk area. Check your Owner's Manual if you have
it, or call a dealer. Unlike Mr. Bat Grunt, many internet users do post
helpful information.

Gyz

You may be able to accezz the catch from inzide the boot - remove the
panel right behind the filler pipe/door

David - who had the NRMA guy do that on hiz car one time