Newbie Telescope Troubles

I have a Nextstar 8" with the goto feature. While trying to sharpen
the focus, I turned the focus knob too much and now I have a big black
circle in the middle of everything I look at. (I assume it's the
shadow from the secondary mirror. I've tried to get rid of it by
turning the focus nob back, but it doesn't help. (The circle moves off
to the side and the image gets smaller and smaller.) What have I
done? I think I broke it. Help!