NADH is oxidized once the electron acceptor accepts an electron from it and oxidizes it into NAD+.
For example:
In alcoholic fermentation, the electron acceptor is acetaldehyde. So it accepts an electron and NADH becomes NAD+.
In lactic acid fermentation, pyruvate is the electron acceptor. It “accepts” an electron from NADH and it becomes NAD+.
Process repeats until oxygen is present.
Hope that helps and have a great day! :)