Getting offline gmail online on a Mac

I use gmail on my Mac (via Apple mail app). It has suddenly gone offline and I've tried

a) shutting down the app

b) restarting my computer (several times)

but it remains offline. I can still download emails via the same - Apple mail - app on my phone so I don't understand.

Is it possible to have a hacking attempt but just on my laptop?

Any ideas?!?