"Mail" does suddenly not pick upp my mails from my gmail account. I can send from gmail and I can pick up from my other accounts.
Log in to Gmail from a web browser and check its settings.
Sometimes you merely have to confirm your password in Gmail's browser interface and Mail will start working again.
In mail I asked for "help" and then I was connected to "get mail" which I clicked and then suddenly all seems to work again.
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