Haworthia's are very tough plants. I can't say what caused the damage but starting over again is easy.
- Cut the plant at the damaged area on the stem
- Leave the cutting on a sunny window for at least a few days to a week (this is to let the stem dry and harden off)
- replant in a free draining soil in a sunny window (chicken grit or other small sharp crushed stone is a good soil additive to ensure drainage)
- during the phase that it is rooting water thoroughly until water runs out of the bottom of the pot when the plant looks dry. (this might be weekly or biweekly depending on conditions)
It is possible the base of the old plant will sprout new growth so don't give up on it. Just keep it in bright sun and wait and see.
Edit: the question was asked whether you can reuse the original potting soil for the plant. This is a judgement call. If there is any possibility of fungus, rot or insects you are better off using a new potting mix. If there are no signs of this you could just harden off the top of the stem on the windowsill and stick in the pot with the old one.