MonoBehaviour.OnEnable()

Description

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

// Implement OnDisable and OnEnable script functions.
// These functions will be called when the attached GameObject
// is toggled.
// This example also supports the Editor.  The Update function
// will be called, for example, when the position of the
// GameObject is changed.

using UnityEngine;

[ExecuteInEditMode]
public class PrintOnOff : MonoBehaviour
{
    void OnDisable()
    {
        Debug.Log("PrintOnDisable: script was disabled");
    }

    void OnEnable()
    {
        Debug.Log("PrintOnEnable: script was enabled");
    }

    void Update()
    {
#if UNITY_EDITOR
        Debug.Log("Editor causes this Update");
#endif
    }
}

Note: OnEnable cannot be a co-routine.
Additional resources: OnDisable.