How to return a value from a void function and use it inside the void loop?

Dude, by seeing the code you have provided I can tell you that you have forgotten to start the serial communication, Serial.begin(9600);
Second mistake, you must call the function. Calling a function means to "trigger" it.
Example:

void setup(){
}

void loop(){
int x =0;
if( x==0){
cheesecake(); //cheesecake() calls the
//function called cheesecake
}
}

void cheesecake(){
//after calling cheesecake you come here
//here do whatever you want
}