Because working offline used to be a thing.
Not getting email, or email not sending? Your Outlook mail client may have recognised that you were not connected to the internet, or couldn’t authenticate to the mail server, and ever so helpfully switched itself into Work Offline mode.
When this is useful:
- Use your mail client without being connected to the internet, chewing up your precious bandwidth or data quota on processes the mail client runs.
- Remain productive while not connected to the network or while your mail client can’t authenticate your connection to the mail servers.
- Go back online when you are ready to send all the email in your outbox or when you think you need to fill your inbox with fresh email.
When this is not useful:
- At pretty much any other time.
<rant>
Why is this still a thing? Like really! For those who actively use it and have a genuine use case for it, more power to you. For the rest of us, this feature needs to go through a rewrite.
</rant>
What you might see:
Or 
Tucked in a very non visible part of your screen (normally the bottom right) and in very small font because we are all viewing this on screen resolutions of 2560×1440 or greater.
What to do:
Windows
Go to the Send/Receive tab, choose Work Offline to reconnect (yes, I agree, clicking Work Offline to work online gets confusing real fast).
Mac
Go to the Tools tab and switch the more useful switch from offline to online – It will turn a nice helpful green.
Other things to try:
Go to https://outlook.office.com and try the online web app version of outlook.
If the Outlook Web App is functioning, use it to complete any pressing tasks before moving through the following steps (reach out to your IT team if you require assistance with these).
- Try toggling the offline status off, then back on again.
- Check for Updates.
- Create a new Outlook Profile.
- Remove your account, then recreate it.
Reference article:
https://support.office.com/en-us/article/switch-from-working-offline-to-online-2460e4a8-16c7-47fc-b204-b1549275aac9
