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When this happens, it is usually the result of a corrupt operating system file. It's not something that can normally fix easily. It may require a full reset of the phone. Before doing that you can try going to settings and then your accounts and removing your Google account and then adding it back. But what I would recommend to avoid problems in the future is going to settings, then going to back up and reset and then selecting factory reset. This will take several minutes to complete – just let it go and complete the process – but it will almost certainly once you do that and will have to go through the setup of the phone again, but then everything should synchronize back. The only things that will not come back are things that you do not have synchronized with the cloud – for example if you have pictures that are not backed up to the cloud, you'll want to copy those to your computer first.