Could be eM Client isn’t reaching the licensing Server, or the Cryptographic services and CNG Key Isolation are not running.
See @Gary post below from the following thread
(Gary’s post)
“eM Client needs to periodically contact the license server to validate your license. If you have a Firewall or VPN that is blocking the connection, then eM Client will eventually be disabled”.
“Can you see if these links open in your web browser”:
https://licensing.emclient.com
https://licensemanager.emclient.com
http://emclient.com
“If they don’t open you need to configure any Firewalls or VPN’s to allow eM Client access.
“If you can reach those web pages, but the license server still cannot be reached, then it may be that some apps turned off specific services that are required”.
So if you have Windows “Click on the Windows Start button and type Services" and Open the Services option from the results”.
Then “Check that both Cryptographic services and CNG Key Isolation are running. If they aren’t, start them”.
Also see this thread.