Galaxy S2, Hangouts app deletes incoming text messages
There could possibly be a bug which is causing the app to Delete the latest incoming messages .


Un-install the application.
Power off and wait for 2-3mins 
Switch ON the device and install the App from the Play store 
Check for and install the latest updates for Hangouts from the play
store


Always keep your applications up to date in order to avoid some bugs like this in the future