Since yesterday afternoon I've gotten the following message when trying to log on to my site:

Temporarily Unavailable
We are offline for a brief maintenance update. We will be back shortly.

Any idea when service will be re-established?

It is and has been back. Try to force it to load the page. That is loading from a local cached copy. If that doesn't work you might need to clear cache on your browser --- see this site for many examples of different browser types https://kb.iu.edu/d/ahic

Ctrl+F5 works for all the browsers (Edge, Chrome, Firefox, don't have Safari), Shift+F5 only Chrome.

Or manually find the settings shown in Link Jessica gave.