Q.need help with my starfish cactus
Over the past week my beautiful starfish cactus has started turning black in areas and dying. This is such a beautiful cactus and I have enjoyed it for 3 years now and don’t know what the problem is our how to fix it. Please help me.
Thank you,
Cindy Jeans

This looks like it could be a case of cactus stem rot or root rot, probably fungal in nature. Sometimes this is caused by over-watering - feel the soil - is it damp/soggy to the touch?
I would recommend that you read the following article and follow the prescribed treatment which involves removing the diseased parts and re-potting the cactus in a clean pot in sterile soil after washing off the roots thoroughly. There is no guarantee your cactus will rejuvenate itself, but it's worth a try.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-going-soft.htm
And, just in case, you may want to consider starting more starfish cacti plants using cuttings from this one.
For more information on growing starfish cacti, please visit the following link:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/starfish-flower/growing-starfish-flowers.htm