According to Mausala Parva: Section 8, Arjuna says to Vaishampayana rishi:
In my very sight, O Brahmana, thousands of Vrishni ladies were carried away by the Abhiras of the country of the five waters, who assailed us. Taking up my bow I found myself unequal to even string it. The might that had existed in my arms seemed to have disappeared on that occasion. O great ascetic, my weapons of diverse kinds failed to make their appearance. Soon, again, my shafts became exhausted.
In some other version of Mahabharath (I don't remember the book name), I read, when the robbers kidnapped the Vrishni ladies, Arjun was trying to use Sahastraastra or Parvataastra, but couldn't invoke the astras. And one among those robbers said to arjun - "The new Yuga has already started and no one now fears your Gandeev, its useless!" Was it true?
Also, in one of the ways that I see, everyone who had the knowledge of Astra where either dead (like Vrishakethu) in some or the other way, or never appeared again (like Lord Parshuram, still alive). Ya I know Ashwathama is also alive and many claim to see him, but I want to know why the knowledge of astras disappeared?
Is it that after Sri Krishna left his human body, Arjun lost his powers? Because Arjun and Sri Krishn were Nara and Narayana, so Nara is powerless without Narayana? (But this is not my question!)
UPDATE: My question is not about kidnapping of Sri Kirshn's wifes. My question is about Arjun losing his powers and why?