The Great Gatsby, knocked down and out

Could you please tell me the meaning of "knocked down and out" in this excerpt from the chapter Nine of The Great Gatsby? What is the difference between "knocked down" / knocked out" and "knocked down and out"?

Dear Mr. Carraway. This has been one of the most terrible shocks of my life to me I hardly can believe it that it is true at all. Such a mad act as that man did should make us all think, I cannot come down now as I am tied up in some very important business and cannot get mixed up in this thing now. If there is anything I can do a little later let me know in a letter by Edgar. I hardly know where I am when I hear about a thing like this and am completely knocked down and out.

Thank you.