Scripting Reference
MonoBehaviour.OnEnable

function OnEnable () : void

Description

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

JavaScript 
    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.