how to power on a galaxy tab 3 that was on charging

Solution
When you had it on the charger/charging did the device actually show that it was charging? If not, it may have not actually charged.

Try holding down the power button for 20 to 30 seconds and then releasing it. Now plug the charger back in. If it shows that it is charging, leave it on the charger (and turned off) for 2 - 3 hours. Then try turning it on.

If it doesn't show as charging, when you plug it in, the cord, battery or port may not be functioning correctly. The simplest one to test first is the cord. Try a different cord on the device and see if it shows charging (you can also test your cord on a different device). If the cord doesn't work, replace it.

If it is the battery or the port that are the problem, they are more...
When you had it on the charger/charging did the device actually show that it was charging? If not, it may have not actually charged.

Try holding down the power button for 20 to 30 seconds and then releasing it. Now plug the charger back in. If it shows that it is charging, leave it on the charger (and turned off) for 2 - 3 hours. Then try turning it on.

If it doesn't show as charging, when you plug it in, the cord, battery or port may not be functioning correctly. The simplest one to test first is the cord. Try a different cord on the device and see if it shows charging (you can also test your cord on a different device). If the cord doesn't work, replace it.

If it is the battery or the port that are the problem, they are more complicated to fix.
 
Solution