MonoBehaviour.OnEnable

This function is called when the object becomes enabled and active.

JavaScripts

function OnEnable () {
print("script was enabled");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnEnable() {
print("script was enabled");
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnEnable():
print('script was enabled')

OnEnable cannot be a co-routine.