First of all, make sure you are in an area with sufficient internet coverage. The app doesn't have an offline mode (yet) and will need good internet to work.
Close the app (normally by swapping up) from your recently opened apps on your phone and re-open it again.
Restart your phone.
If the above doesn't work log out from the app and log back in. You should receive a login link in your email.
If you still have issues contact the Kantan support team by messaging in the app or by emailing email@example.com