The most common cause of this issue is the internet connection being interrupted while the app is loading. Reset the app by closing it (swiping it off the screen on iOS and Android) and relaunching it.
If you have been beta-testing your app via hockeyapp, or you have had a few versions sent to you by the Plattar team, sometimes old data can be left from previous versions that will prevent loading. To fix this, fully delete the app and reinstall from the appstore (or hockeyapp, if you are beta-testing).
If the issue persists, and websites load fine on the device contact a Plattar support team member.