How can I unlock a door to an old buffet?

Do you have a building supply salvage yard near you? The one near me has a huge collection of old skeleton keys in dozens of different styles.

Can you dismantle the hinge? If that doesn't work maybe call a locksmith

If you can see the manufacturer's info, googling that may help to track down what type of lock/locking mechanism it is.

https://www.hunker.com/13410859/how-to-unlock-an-antique-without-a-skeleton-key

https://www.art-of-lockpicking.com/how-to-pick-a-lock-with-a-bobby-pin/

http://ideas-6.blogspot.com/2016/02/unlock-antique-without-skeleton-key.html

If all this still fails, you can either call a locksmith, take the piece to a locksmith (cheaper than a house call) or purchase a "warded lock set" that should get you in.

Can you not take the back of the cupboard off to get access to the lock inside. Remove the lock or try and find a key

Try sliding a credit card or something thin and sturdy around the sides to see if you can move the bolt. If not cut into the back and try removing the whole door and lock.

Try contacting an antique dealer. They may be able to help.

maybe try a flat head screwdriver?

Everyone has great ideas. I would try removing the back of the cabinet first. It seems to be the simplest answer if it is possible.

Remove the back so you can see what your dealing with. :)

I agree with removing the cabinet back first to see how the mechanism works. If saving the lock is not a problem you can drill it out then replace with a universal mechanism.

i would gently remove the back of that cabinet and then remove hinges so lock let go