Updated June 13th, 2019.
Sometimes, the Mac’s Mail all of a sudden doesn’t work. You don’t get any mail. When you look at the Inbox you see a little lightning bolt, as shown below:
The lightning bolt is something you can click on and get info from. Usually it means the account or accounts you’re checking are, for some reason, offline. We aren’t concerned (this time) with the reason. We just want to fix it. Try going to the Mailbox menu and choosing “Take All Accounts Online.” That ought to do it.