I experienced the app not connecting to the car for 3 or 4 days, after months of being fine.
Very frustrating as I really needed it to work over the weekend. I waited days for servers, I tried all the normal phone related solutions. Nothing worked.
I then tried rebooting the infortainment system ( home key for 10s ), no difference
I then reset the car( disconnect negative lead on 12v battery ). Ping! it all started working again.
I have found that generally resolves little niggles quite consistently