Same thing happened to me many years ago on my then brand new trailer (see sig)
One of the pins in the lock has become stuck. Until you unstick it the lock will not open.
We called a locksmith out to the campground at 10:30 at night, who tinkered with it for a while then got a hammer and beat a slotted screwdriver into the lock, ruining it but getting us back into the trailer. Rockwood warranty sent me a new lock and paid for the locksmith visit.
You might try sticking something in there to work the little pin loose. Or tapping on the lock with a hammer. Or do what our locksmith did.
The locksmith asked me at least 3 times if we could crawl through a window to get the key. I told him 3 times I have the key in my hand!!
One of the tricks he tried (that didn't work) was putting the key in the lock and tapping it gently with a hammer. But you might try it anyway.
Best of luck!