I can no more see the tracker's history

Maybe you have de-installed the UrbanTracker app, and then re-installed it; or you have paired the tracker to an UrbanTracker app on another mobile phone. In both cases the new app is not aware of the tracker's past history.

I have wrongly enabled flight mode

If you want to exit the flight mode just after enabling it, simply take the device, turn it OFF and then turn it ON by the ON/OFF key.

I see numbers instead of an address

These are couples of geographic coordinates, identifying the exact position of the tracker. Maybe in that moment the service was not able to receive the corresponding address. Check the map or the location history later, and they will be "translated" into the address.