The app was removed a few weeks back due to technical issues but is back now and can be downloaded
A few weeks back, Whatsapp completely removed their app from the Windows Phone. It was due to some technical issues and some serious problems that the app had to be entirely removed and there was no mention of when things would get fixed or when the app would come back on the store again.
However, it seems that things are still under the way since Whatsapp and Microsoft have been looking into this issue together and were trying to fix whatever had to be fixed. Though we are not so sure about the problems which existed and Whatsapp gave away no information regarding what went wrong but that doesn’t concern us any longer because the app is back on the store now.