Why has my old iTunes account disappeared?

Years ago, iTunes forced me to open a 2nd account, then, of course, I was stuck managing 2 accounts as so many of us experienced. With my original account I had purchased hundreds of songs and apps. I have always been able to sign in to that account and download the content to a new computer. Today, I am unable to do so, even though I recently downloaded quite a bit of that old content (as recently as this summer).

Where did my account go? It's the one I'm using right here on the community forum to ask this question. As mentioned, I used it just this summer to access my content, representing thousands of dollars of purchases over many years. Why can't I sign in to iTunes to download any of that content?