First try getting someone to push hard or shoulder the door from the inside whilst you open door handle from the outside and then same again whilst trying the inside door latch. If this does not work, I would suggest that you have a mechanical failure of the lock mechanism. I've no experience to offer on your particular car, but repair would involve removing the door card (internal door trim) to expose the varoius rods and levers inside the door shell. This will be difficult with the door closed but should be possible eventually. The worse case could involve destruction of the door card and need a replacement sourcing from eBay!
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