Fortnite is currently being updated and when the servers are back online players will be able to update their game clients and drive straight into Fortnite's Season 4.
Patch 4.0 has debuted and a disaster has happened, the rumored comet crashing onto the map has become real and many locations have been affected by its impact. This has resulted in many locations being completely demolished or changed.
Also included in the new patch is updates for smoothness for characters movements on the starting island as well as performance improvements all round and many more patches/bug fixes. If you are interested in viewing the full patch notes visit the official Epic Games v4.0 patch notes here.