Functions? Creating a Radio Hub

You have messed up the last code that you posted by ending the loop() function too early,

Here it is Auto Formatted

unsigned long startMillis;
unsigned long currentMillis;
const unsigned long ScreenRefresh = 5000;

void loop()
{
  currentMillis = millis();
  if (customKey == '*' )
  {
    currentMillis = millis();
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("The Coop Door Is");
    ScreenTwo = HIGH;
  }
}

if (ScreenTwo == HIGH)  //if we are showing screen 2
{
  if (currentMillis - startMillis >= ScreenRefresh)  //if the period has passed
  {
    CoopDoor();  //refresh the screen
    startMillis = currentMillis;  //save the time that it was refreshed
    ClearScreen();
  }
}
}

void ClearScreen ()
{
  ScreenTwo = LOW;
  lcd.clear();
  MainMenu();

and it ends without a }

A couple of other things
1 - you only need to update currentMillis once. Usually it is convenient to do this at the start of loop()
2 personally I would use a boolean for the ScreenTwo variable as the code reads better, at least to me

if (ScreenTwo == true)  //if we are showing screen 2