I had the same issue... I tried all the other suggested ideas and it didnt work so this is what I did and so far its working pretty good. I pressed both volume up and down buttons and then the power button for a few seconds and the android screen popped up. I pressed the down volume button and it restarted after I put my PIN in.
I have a Tab A that when I go to use it, I put in my pattern, and it asks if I want to power down or restart. I choose restart, but it doesn't restart. It just goes back to the lock screen. The samt thing happens if I choose power down, or tap off to the side to abort restart. I can't get in at all.
I just had the same problem. It frustrated me so much that I just threw it out. I think it was either because I left it running and never turned it off or an app caused it to have this problem.
I like most of Samsung's hardware but this thing was a piece of rubbish (why no backlighting on the touch sensitive controls, it was amazingly slow and the WiFi speeds on it where hopeless (even though all the other devices I've got which use WiFi are quite quick)).
To me it's no loss.
I rebooted into Safe mode, removed my password, restarted my tablet and its working again
To reboot in safe mode
Hold down power button, click on power off, then hold that button down until it starts again in safe mode
Another potential fix, below:
I experienced this issue today, and nothing listed here helped.
I was finally, consistently able to get my Tab A 8" device to (non-destructively) reboot, using the following process. No data was lost, nothing was cleared using this process, etc.
Good luck, hope this helps.