It is hard to tell from your description. At first guess, I would pick something like damping off--soil too moist. Depending on your climate in India, they may not be getting enough sun as well, with too much heat.

Powdery mildew isn't always from heat (though your probable humidity will contribute) but more from lack of airflow--very common with indoor herbs. Even my rosemary and thyme in dry Colorado are prone to it. A fan can help, or just wash the leaves off. I also use neem oil when it gets bad.

A closer up pic of the affected basil leaf may help--most of the plant looks healthy. I've never had baby chives drop that way--nothing regrows? Sometimes you will lose some, but more will grow. Do you know anyone else with mature chives that would give you a division?

Thank you balloonflower.

I'm sorry about the picture

Another thing that I should have mentioned was that even though I grow them on my window, they're on the outside of the window. They're not behind glass. I can't really fit a fan there but I'll try the neem oil, thank you :)

Unfortunately I can barely even find people who have heard of these herbs and know what they look like, let alone people who grow them. Even the plant nurseries here don't grow them. I've been struggling to grow rosemary from seeds too.

I see some kind of leaf miner damage.

thank you floral. I looked it up and i have seen some of those leaf miner flies hanging around the soil of my plants.