you can get rendered unable to attack for a short duration by enemy skills. In most cases you will see thorny vines around your feet (I'm carefull here to say always). If this last longer then a few seconds it is a glitch.
My level 48 DK just started experiencing a similar problem. It started last night with the dragon ultimate power where instead of zooming at the enemies in a flash, my guy would be running about, (taking tiny, tiny little steps, oddly) while wearing dragon wings and be unable to attack or block for 20 seconds.
It would have been comic if I hadn't been on the 4 man pirate island pledge at the time.
I also had a 4 second lag on any action button (fighting, looting, conversations, everything).
So, I equipped a different ultimate and reported the issue.
But NOW, well now the guy won't even draw his sword half the time and if he does, he then won't swing it about. It's like he has no stamina at all. Either the dude stands there like a stump while mobs break his armour and then kill him or, he draws his sword, swings it and it connects with ab-sol-utely nothing and the mobs show no reaction as if they can't even see him.
The action button lag is now about 10 seconds, when they have any effect at all.
I just got through part of a quest with block and ardent flame and a heap of repair kits but now I just keep dying so,
Man, I've logged off in a depression. I guess PTSD was inevitable considering what he's been through.