Q.Why are my spider plant’s leaves turning brown, not at the tips, but at the base where they come out of the plant?
This causes the whole leaf to die. Even leaves on the babies are turning brown at the same place. What is causing this? How can I help it? It was very healtyhy, watered with purified water only as it is an office plant in the city.

Certified GKH Gardening Expert
These are very drought tolerant and will not tolerate moist soil for very long. This will cause an infection in the soil. This is likely the case now. If at all possible, you will want to repot with fresh soil and replace 1/4 of the water next time that you water with hydrogen peroxide. This will help kill off any remaining infection. You should only need to do this once.
https://www.gardeningknowhow.com/houseplants/spider-plant/treating-diseases-of-spider-plant.htm