It would still be hard to get up, but a fallen knight wouldn't be rendered as helpless as a turtle on it's back or anything. A suit of armor weighed between 40 and 60 lbs. So imagine getting up with that much extra weight. It will slow you down but it won't pin you to the ground.
What a lot of people seem to forget/neglect is that full suits of armor weren't just mass produced pieces of metal slapped together for anyone to wear. If you were putting an investment into these (which a full suit most certainly was) it was custom tailored to the shape of whomever it was for. In a properly fitted suit of armor, a person could move seamlessly. It wasn't as clunky as so many more modern sources make it out to be.
