In glycolysis:
2 ATPs put in and 4 produced by SLP = a net of ...............................2
2 NADH2 go to ETS where 3 ATPs produced/NADH2 .......................6
In the Krebs Cycle:
via SLP .............................................................................2
8 NADH2 (-> ETS where 3 ATPs produced/NADH2...................... 24
2 FADH2 (-> ETS where 2 ATPs produced/FADH2) .......................4
Total ...............................................................................38
When a cell carries out fermentation only glycolysis occurs. Under these conditions the only ATP that the cell obtains is from SLPAs noted above, 2 ATP are put in and 4 are produced resulting in the net production of 2 ATP.
It becomes reoxidized by donating its 2 hydrogens to an organic molecule - as an example it can donate its hydrogens to pyruvic acid and convert it to lactic acid. The pyruvic acidhas become reduced (to lactic acid) and the NADH2 become oxidized back to NAD.