An App disappeared after downloading, Where is it?
It appears there is an issue with your phone with Google Play not installing correctly. 


If the app is in Google Play you can't install it manually 
When you go back into Google Play and search for the app you have installed, do you see the button Install or Open? 


If the buttons Uninstall &amp; Open show the app is installed 
if the button Install shows you will have to reinstall