There is a good chance your Hawke doesnt have enough points invested in your main stat (warrior=str, mage=int, rogue=dex) which means your attack rating is too low.
I suspect if you turn on damage popups youll see that you're just hitting for 1-3 damage
So I need to raise your main stat to solve the damage-hitting-problem.
By testing if it had to do with too few points in my Hawke's main stat=Str.
But even when I used a trainer to try and solve the damage-hitting-problem by maximizing my strength stat to 100, he still wouldn't deal any damage (I know this for sure, because I had enabled the 'Show damage numbers' option).
Has adding more points to your main stat solve the problem for you? Because I can't solve it in any way :(
I want to finish the story (at least the first time) without cheating or that only Hawke's companions are able to fight and I have a useless main character :S