Unfortunately, you will have to directly contact the new FC leader.

Since leadership was automatically changed by the system, the new leader now owns it and unless they willingly hand it back to you, there is nothing you can do.
I would note the leaders name and travel to the DC and World they're on (can check by clicking on their name in the FC list) and do a search.

Otherwise, you might want to leave that FC and start a new one.
Just remember that leadership is automatically passed after 35 days of not logging in.