Banging it, restarting and rebooting (holding power button down) did not work for me.
I downloaded an antivirus software and cleaned out my tablet. It asked me to remove some possible apps (mine was towelroot). After cleanup, I restarted my tablet and auto rotation WORKED AGAIN.
Maybe it was a virus or a corrupted application that helped me once before to connect my keyboard. But anyway, after removal, my keyboard and all the functions on my tablet worked.
So try using the antivirus and restarting it as well.