Your email sync can become unsynced if something chnages on your email provider's end. For example, if you've changed or updated your gmail/ outlook password - this would cause your email to stop syncing in Loxo.
To resolve simply click on the resync button.
Navigate to Settings by clicking on your initials icon in the bottom left corner
Click on Synced Emails on the left-hand side.
Check to see if your email is synced. Simply hover over the email you want to check. A correctly Sync'ed email account will show "running" as the sync state.
If yours says Invalid, you'll need to resync.
Click on the resync button