If you open the Batt door in the trunk, you should find a release cable.....then find out why this happened...thinking frozen lock or something.

 

You can also disconnect the wire to the gascap lock.

 

If the car doors were locked the tank door locks as well.

 

You may want to search forum using keyword "gas door actuator".

 

Yup, failed fuel door lock actuator. Common. Can be fixed DIY but PITA. Pricey part also. Just leave it unplugged.

 

failed fuel door lock actuator. Common

Interestingly, this isn't one of the top-30 failures as currently listed for the E39 (should it be added?).

Also interesting is that nobody has suggested a best-link reference for it in the VERY best of E39 Links.

So, to the OP, your job, should you decide to fix this problem, would be to do the research and let us know which were the most helpful gas door actuator lock threads.