https://www.gamespot.com/articles/fortnite-servers-down-for-maintenance-update-compl/1100-6460671/

A new update is on the way soon for Fortnite: Battle Royale. Coinciding with the game's first anniversary, Epic is rolling out the v5.10 patch on all platforms--PS4, Xbox One, PC, Nintendo Switch, and iOS--today, July 24, which will kick off a special, limited-time birthday event and introduce other new content to the battle royale game. [Update] The servers are now back online. You can read the full Fortnite 5.10 patch notes and our Fortnite 1st Birthday challenge guide to see everything that's new and what you'll need to do to get some fresh cosmetics.

As usual, the v5.10 update is preceded by a period of server maintenance, during which time the game will temporarily be taken offline. As with most updates, Epic hasn't revealed how long the maintenance will last, but it started at 1 AM PT / 4 AM ET / 9 AM BST. In most cases maintenance lasts between an hour or two.