In MMA how a fighter be knocked out in he didn't lose consciousness and was down for only a second?

The fighter got punched and went face down but he wasn't knocked out and jumped up 1 or 2 seconds later and still wanted to fight but by then the referee had already jumped in and stopped the fight ruling TKO.How can he be knocked out without him being out and can he change the ruling.This makes no sense to me.If he still wanted to fight why not let him.