All is well with your citrix farm and then you receive the below log within your event log.  

Error: MetaFrame Presentation Server has entered the grace period. You have xx hour(s) remaining before this server stops accepting connections from client devices.

This means that your Citrix server can no longer communicate with the license server but will still allow users to connect until the grace period ends.

Check that the servers are up and running and the required services have not stopped. Carry out your basic checks such as pings.

Note that the grace period his held in memory on the Citrix server(s) so if the Citrix server(s) are rebooted, the cached license key may clear stopping users from connecting.

A fix which i carried out recently was to restart the Citrix licensing services on the license server. After a couple of minutes you should see events appear within the application/system log indicating that the servers have left the grace period and are now able to talk to the license server.

Carry out further tests to make sure users can login.