Key will not unlock the door

Does the key turn at all when you put it in the lock?

Are you in an area where it is below zero? If it is freezing then the lock may have water in it. The common method uses heat to thaw/dry the lock. A hair drier or similar kept back enough that you do not damage the paint. There are also special tools that are basically a copper rod that inserts into the lock to thaw them.

If you are not in freezing temperatures either the key is damaged/worn. Or more likely a spring or pin inside the lock itself has broken. A locksmith should be able to deal with either one.

If the key turns but does not unlock the door, the likely cause is that the rod going to the actual latch has come loose or broken.

Call a locksmith, they will be able to use special tools to open the door. It is not a hard job but without the correct tools you can damage expensive things. They will want to see proof that you own the vehicle.

Ask what they require as proof when you call. They may call the former owner or even run the VIN number to verify ownership before they will touch it.
(I know I would, not that I do not trust people, just making sure I do not end up in jail for grand theft auto)
Or if you are not in a big hurry, get the paperwork done and then call. That way there are not any issues.

As for the locks themselves, you may need a new key or the radio that interrogates the key to enable the locking system might be bad. Hard to say. That system is a bit complex compared to a fully manual lock. You can do a lot more than just unlock the car.

Good luck.

It will depend on which locking system that car has. They use a central locking controller and different parts depending on the exact build date, VIN number and options. Most of the information I have access to is somewhat more generic than what the manufacturer has.
One source says it has a manual bypass, another says it does not and a third says it depends on the options. Once you have it open or the smith might be able to tell you when they work on it exactly which one you have.