If you use Google or Yahoo accounts your accounts might have stopped synchronizing after the upgrade. This is most commonly caused by external apps like Antivirus, Firewalls, VPNs, etc. which are blocking the new eM Client 8 for your safety because the program is new and therefore not in any trusted databases of these programs.
We have seen this often happen with Avast, Kaspersky and AVG, though others could cause the same problem.

Adding eM Client to exceptions in these apps and disabling any mail shields should resolve the issue.

We also recommend reporting this to the provider of your security app, so they can update their records and not block eM Client in the future.