Help android internal storage is missing

Hi I have had this Vodafone smart basic android for 2 years now and it use to have around about 6 gb internal storage. But now its just over 2 GB even when I reset it to factory settings and I can't download anything else.could you tell me how I can fix this without downloading a app or rotting the phone? Not even a SD card can save me as I can't download anything from Google play to go there by default.any help would be much appreciated thanks.