Make sure the tablet is charged. Then hold in the power button (only) for a minimum of 15 seconds. This will force a hard reset but no data should be lost.
If that still doesn't work, you could try a factory reset, You will lose all your apps and data (except on SD card) if you do this.
1. With the device off, press and hold Volume Up and Power.
2. Release the Power button when you see the Samsung logo, but continue to hold Volume Up until the recovery screen appears.
3. Use the Volume buttons to navigate the menu and select wipe data / factory reset. Press Home to choose the selection.
4. Press Volume Up continue.