If for some reasons like a server shutdown or the machine being powered off or internet failure etc ... the server stays offline for more than four days, when it's back online it should be able to connect to the license server to check the license. Otherwise, when this is not possible for example because of a DNS or an internet failure, the server will issue message about "Subscription expired".

To make the server check the license immediately and restore its operativity, restart it via UI or from command line:
nxserver --restart