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This usually happens if there is any issue with the firmware or if the internal board has failed. We may need to factory reset it but lets try to reboot it first.
Turn off the Tab
Press and hold down Volume Up and Power buttons at the same time for a few seconds.
Android system recovery will be loaded now
Select wipe cache. (Volume buttons to go up or down, select with Power button)
Then select reboot system now and press Power. This will reboot it
If that didn't work, then you will have to factory reset it. Factory reset will erase everything from it and install a fresh android version. This can't be avoided if it is not booting at all.
Boot into Android recovery again (using the above method) and select wipe data/factory reset. (Volume buttons to go up or down, select with Power button)
Select Yes — delete all user data.
Then select reboot system now
Please let me know if that didn't work, so we can install a new firmware/software on your tab from your computer. I will send you instructions.