Reports via Twitter started appearing around 12 pm GMT on 6th April, with players reporting receiving an "Invalid Client" error when attempting to login.
Epic confirmed the issues at 1:30 pm and later, at 4:30 pm, added that the Epic Games Launcher and Fortnite servers were still "degraded".

Players are also reporting issues with their friends' list, with other players not appearing even when they are online.
Fortnite Status on Twitter Our teams are still working on solving these issues for all players. Logins to the Epic Games Launcher and to Fortnite are still degraded.— Fortnite Status (@FortniteStatus) April 6, 2021 Twitter
As informed by the developers, the services should be up soon, and players should have regular access to these applications soon.
We will update this page with any news as we get it.