The S6 (I'm assuming you mean a Samsung Galaxy S6) was originally released with Android 5 (Lollipop) but has since received updates to ver. 6 (Marshmallow) and 7 (Nougat). If you updated your phone to either of those versions, the default USB mode is 'charge-only'. When you plug it in, you must pull down the notification bar at the top, select USB options (or whatever Samsung calls it) and set it to MTP mode ... every time. You can't now set it so it always connects for file transfer unless you flash a custom rom that supports it in later versions of Android. It's a security thing.