Update at 14:50 UTC - We are seeing reports that this issue has been resolved, and players should once again be able to launch MSFS 2024 & MSFS (2020) as normal.
We are aware of players reporting being unable to sign-in and launch both MSFS 2024 and MSFS (2020) on PC.
Please see the XBOX Live Status page for updates from the XBOX team regarding this issue.