Code or Compiling error

I tried to get the code to run but I keep running into Error: In function main: 46: Undefined reference to loop
error: ld returned 1 exit status

#include <avr/io.h>
#define F_CPU 16000000
#include <util/delay.h>
#include <avr/interrupt.h>
void setup();
void initint0();
void loop();
void setup()
{
  DDRB = 0b00111000; 
  DDRD = 0b11111011;
  PORTD = 0b00000000;
}
void initint0()
{
SREG |= 0b10000000; 
EIMSK |= 0b00000001; 
EICRA |= 0b00000001; 
}
ISR (INT0_vect) 
{
  int PButton=digitalRead(2);
	if (PButton == 0b00000000)
  {
    delay(3000);        
    PORTB = 0b00010000;
    delay(3000);              
    PORTB = 0b00001000;    
    delay(2000);              
    PORTD = 0b01000000;    
    delay(8000);
    PORTD = 0b10000000;
    delay(2000); 
    
  }
   else    
   {

    PORTB = 0b00100000;
    PORTD = 0b10000000;       
   }
}

I’m not sure if this is a issue regarding how the code is structured or another issue. If you have any suggestions on how to improve my code please let me know.
This error happened after trying to insert a interrupt

Your code has a function prototype for the loop() function but no actual loop() function.

You need to add one, even if it is empty

Change void loop();
to

void loop(){}