I recently tried to login to a service which uses the Google Mobile App for 2 Factor Authentication and it suddenly stopped working after no apperent change.

The account was still activated on my phone. I had rcently I travelled to a city with -1  hour time difference and the Time Zone had been set incorrectly back on my phone so I had to set it to manual. 

Since generating 2FA codes relies on an agreed time between your device and the third party server, having the wrong time (or timezone) on your device can cause incorrect codes to be generated, which might explain why you’re having difficulty logging in.

Fixing the Automatic Time on my phone fixed this!