My gas tank door will will not open on my Mercedes benz s500. how can i get it to open

Usually on remote opening gas doors there is a second latch for it sometimes in the glove box. the manual or a delaership can tell you over the phone if it has one and where it is

If doors are locked the gas door is locked

To open the fuel door you need to unlock the car with your key remote . If the car is locked with the remote the fuel door will not open

Buy a ford. Then you will not have this problem.

The spring in the door is probably weak. If you can find a piece of wood that is wedge shaped and push it into the seam of the gas door then press the release button it should pop open so you can inspect the spring. If you can bend the spring the opposite way then spray the hinge with WD40 it should work for quite a while