You may have to start over with the divorce.
You may have to serve him by publication.
Is he alive?
Is he in prison?
Is he out of the USA?
Is he in a coma in some hospital?
Has he joined the French Foreign Legion?
Perhaps he's become a member of the feared, ferocious ISIS fighting forces?
Maybe he's gone off the grid in Alaska?
He might just be hiding from you, laughing.
It's anyone's guess where he might be.
Those are questions you need to answer for yourself, not for us.
You probably need a lawyer, if he can't be found by you, or he can't be served.
I suggest you retain an attorney, or at least talk to a couple attorneys.
After a couple consultations and investigations, you can then decide.