iTunes Store Switching Countries

I have a US iTunes account and was trying to purchase an episode of a British show I have watched on Nat Geo channel; when I tried to purchase, a pop up instructed me to switch to the UK store to buy it. I said ok but obviously should not have agreed. Since I don’t have a UK account I wasn’t allowed to buy anyway AND NOW I can’t get back into my US store so I can’t download what I already own! Please help!