Used TalonP tank will not take air!

I decided to take my new Condor, fitted with a TalonP tank, out for a shooting session a few days ago. I was having no problem filling it, but could only get up to about 2600-2700 psi, before my Benji pump could not keep up.
To make a long story short, after about the fourth fill, I heard a clicking sound from my tank, and could no longer introduce air into it. NOTHING!
The tank holds air like a boss, but I cannot get air to go in, even with my new pump I got in the mail, today.
I have never worked on an Air Force tank, and do not have the necessary tools and, of course, the necessary wheelhouse of skills to make the right diagnosis, although i am guessing, quite elementary, that it is the fill nipple “valve” that has become stuck?
I don’t want to venture trying to fix this gun, but do not want to spend a princely price getting it fixed, either. Where might I send it to for having it fixed? I am sure that shipping will be the most expensive (I am in Illinois), so I have contacted a shooting buddy up north from me, and a buddy in Ohio, who has a friend who works on NInja tanks.
Any idea what might be ailing my tank? It date of manufacture is 9.13, I believe.