theme changes back to default by itself

I have a site and ever other day it returns back to the default template some times over night i have tried installing a fresh word press and no change i have also tried removing the default template to see what happened there and that ended up giving me a blank site i seen another post in here about this but they never really got an answer as to how to fix this problem i only seen installing the most up to date word press but theres one problem with that for me i already have the latest one so im stumped please help me fix this