Hold down the power button, until it turns off and restarts
Hopefully it's just being mean.
I
If you're still having issues you may need to do hard resets and the like (remove sd card first)
Not entirely sure what to do but there's been a few posts on these very forums about holding power button and volume down, id only do it as a last resort...It's up to you...do a quick search (samsung tab reset etc...)