I'd like to add :
There are two types of serves offline reasons :
1- Maintenance ; This happens with a notification to all players via system message ingame with downtime. ETA is also given with an announcement.
2- Server Crash ; Server crashes are impossible to predict hence no notification can be given. When an issue like this occurs, only certain staff members can bring it back up. No ETA can be given about when or how since it happens randomly and those staff members are unavailable at the moment or the reason of the crash requires looking into.