whay may wifi don tur on samsung tablet SM T 800

Is the option totally greyed out? Or do no WIFI networks show up when you turn it on?

My first thought would be to back up your data then factory reset the device and see if that resolves the issue.

If it is still having the issue after that, it may be the actual chip on the board of the phone malfunctioning. Getting another chip may prove to be non-cost-effective but more than likely your issue is software related, which a quick factory reset, or even a complete software reload should resolve the issue.

Here are the instructions for factory reset on your tablet (this is for the 8.4" but it should be the same as yours): How to Samsung Galaxy Tab S 8.4 Factory Reset