How would fighter jets make contact with a previously hijacked aircraft? For example, a plane is hijacked but civilians regain control, they know how to change the plane's frequency but no procdeures - i.e. dont know guard frequencies. There are fighter jets alongside who do have contact with ATC. What could the jets/ATC do to let them know what frequency to use?
Nothing. There is no possible world in which someone has the necessary training, knowledge, and experience to operate the complex communication equipment, yet doesn't know 121.5.