I recently upgraded to Version 8 and since that time, I have received no new emails, etc. I now get the warning (red triangle) that the Connection Failed. All of the previous settings to Gmail were preserved during the upgrade.
Remove the Gmail account from eM Client, then add it again.
Removing the gmail account did not work for me. But … I use Avast antivirus, and post Connection failed- red triangle mentions disabling Mail Shield. Disabling that allowed the connections to succeed for me.