Linker error

[andrew2ec]

I'm working on a project in dev-C++

#include<iostream>
#include "Critter.h"
#include <time.h>
#include<string>

using namespace std;

size_t strftime(char* ptr, size_t maxsize, const char* format, const struct tm* timeptr);
int main()
{
    Critter GetOne;
    Critter c1;
    c1 = Critter(0, 1, 2, 3);
    time_t rawtime;
    struct tm * timeinfo;
    char buffer [80];

    time ( &rawtime );
    timeinfo = localtime ( &rawtime );

    cout<<"\t\t*****Take Care of Your Electronic Pet - Critter*****\n\n";
    strftime (buffer,80,"%I:%M%p",timeinfo);
    puts (buffer);
    cout<<"Critter Caretaker\n\n";
    cout<<"0 - Quit\n";
    cout<<"1 - Listen to your critter\n";
    cout<<"2 - Feed your critter\n";
    cout<<"3 - Play with your critter\n";
    cout<<"Choice: \n"; 
    cout<<"        \n"<<c1.GetZero();


    system("pause");
    return 0;
}

   but I'm getting the following error msgs.

  [Linker error] undefined reference to `Critter::Critter()' 
  [Linker error] undefined reference to `Critter::Critter(int, int, int, int)' 
  [Linker error] undefined reference to `Critter::GetZero()' 
  ld returned 1 exit status 

Can anyone help?