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Explanation: The sport being referred to here is boxing. In boxing, if one person is knocked down, the referee will count up to a certain number (say ten), and if the person knocked down is unable to get up at the end of the counting process, his opponent is the winner by KO (knockout). If the person is able to get up, however, the fight continues.
In your text, the person concerned is probably in a very bad situation, such as being on the brink of ruin or bankruptcy. At any rate, he has suffered some sort of metaphorical blow that has badly "knocked" him.