Hello and welcome to HTS! I have the same problem sometimes when Wi-Fi is weak or disconnected. Try:
Double-clicking the HOME button (the one on the same face as the screen) VERY fast. Your tablet should display all open apps in their separate windows. Find the blank HTS window/app and swipe it off the iPad's screen to close it.
Open SETTINGS, and make sure you're connected to the Internet through Wi-Fi or Bluetooth
Tap on HTS logo to open app.
Making sure the screen is not zoomed, enter logon credentials.