If oxygen is lacking, how might cells meet their needs through fermentation?

They no more respire aerobically (in the presence of oxygen) but anaerobically (in the absence of oxygen). You may want to Google these terms.

Here, the glucose is not directly converted to carbon dioxide and water, but to lactic acid in animals and to ethanol (alcohol) in plants. Less energy is produced in that process.

In the fermentation of yeast for example, ethanol is produced.

Of course, if too much ethanol is produced, the yeast will die since concentrated ethanol is toxic to it, as is lactic acid to human.

Lactic acids firstly produce muscle cramps in human beings.

If oxygen is supplied later on, the lactic acid or ethanol is then broken down into carbon dioxide and water, liberating energy again.

I hope it helped! :)