OS missing. Please help me

Did you try to do a hard reset on it? A hard reset is using a button combination to initiate a factory reset on the device. I would Google your tablet and look for the hard reset combination, on Samsung devices it is generally volume up, power and the home button and than release the power button when the Samsung logo appears. If that doesn't work I would bring the tablet to a Samsung tech. They are able to reload software on Samsung devices. I know some Best Buys have them, I am not sure where else they are. Hope this helps!