This issue can happen if there's an issue with your internet connection while the app is downloading the map style. Please follow these steps to fix it:
Ensure your iPhone is connected to the internet, either
via your mobile carrier or my WiFi. The app can work offline, but
needs to be connected when you first launch it so that it can
download the map data.
On the home screen tap the info (i in circle) button in
the top right corner.
Scroll down and tap the "Reload Map Tiles and Style"
button and choose "Yes" in the alert that pops up.
This will cause the app to quit; when you relaunch the
app by tapping the icon on the iPhone home screen, it should try to
download the map data again.