Update: Its is now available in Store (Via: Plaffo)
Facebook Messenger Universal App is currently not available for download in Windows Store since past few hours. It seems that the app is currently not available on both Windows 10 Mobile and Windows 10 PC devices.
We assume that the last update for Messenger caused this issue and we hope Facebook will bring back the Messenger app pretty soon. We will let you know once the app is back, do let us know if you are able to download Messenger on your Phone or PC in the comments below.