Occatinoally, when you follow a link to the App Store app (or iTunes Store) it will switch to the country/store that the link came from.
To change back to your country's App Store:
Open the App Store.
Choose the Featured (first) tab.
Scroll to the bottom of the page.
Tap where it shows your Apple ID (or says sign in).
Choose to sign out. (or skip this step if you were not signed in)
Once signed out, choose the button again and Sign In with your Apple ID.
I haven’t tried it myself, but I’m told that you can also sign out and back in under Settings > iTunes & App Store; with the benifit of reseting both apps back to your own country.