Konami launched the open beta for the Windows PC version of Metal Gear Online just yesterday, but it's already been taken offline. A developer posted to the game's official Steam Community forum to explain that access to the beta would be temporarily removed due to a "possible exploit."
The development team is currently working on a fix and will restore access once the problem has been resolved.
Metal Gear Online has been available on PlayStation 3, PlayStation 4, Xbox 360 and Xbox One since October. It serves as the multiplayer feature of Metal Gear Solid 5: The Phantom Pain; read our review of that game for more on its online component.
Update: Access to the beta has been reinstated, as confirmed on the game's Steam forum last night.