Check bluetooth, and your sensor and transmitter status, close the app by swiping up and then re-open.
First of all please check that you've got bluetooth on - we need this to be able to send data!Then check your sensor and transmitter status on the app to make sure they are connected and charged, and the sensor is not getting towards the end of its life. If they are fine, then close the app by swiping up, and then reopen it. Your data should then appear.