Q: My beer started fermenting, then stopped, and then started again?
A: This usually happens when the beer goes through temperature changes.
The yeast stops fermenting when it gets cold as the metabolic activity of the yeast slows down.
It then proceeds to start up again when the fermenter warms up.
If your fermentation gets stuck warm up the beer to the perfered temperature range for your yeast.
Sometimes it helps to give the carboy a swirl.
DO NOT add oxygen.