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It would help if we knew what type of plants you are referencing.
Though all plants need sunlight, nutrients and water there can be many differences in these basic needs.

https://www.gardeningknowhow.com/houseplants/hpgen/basic-care-of-houseplants.htm

I'm sorry, but your photo did not come through. It will be hard to say what happened, but usually overwatering, then mistakingly overwatering again to compensate will start the process. Feeding will put more stress on the plant, causing it to shrivel and die.

If the soil is not completely bone dry and the plant is limp, the first thing will be to make sure that the plant has time to dry out quite a bit between waterings (depending on the exact plant)

The next thing to check will be cold tolerance of the plant, and how low the temps are around it.

Too much light can be another cause, but not usually.

Here is an article that will help you to water plants in the future: https://www.gardeningknowhow.com/houseplants/hpgen/how-to-water-a-plant.htm

Here is an article that will help you to use fungicides properly: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Do you have the cultural requirements for the plant? If it's tropical, it may be a full sun plant and not getting enough light. You can use grow lights or fluorescent if you aren't already. Or move it to a sunnier location like a room with south facing windows.

Also, it may be getting too much water. Most plants benefit from letting the soil dry between waterings. Then water till it runs out the bottom of the pot and remove any standing water. Do not let the soil be saturated.

If you think your plant is overwatered, then quickly repot in fresh soil. You may be able to save it.

My best advice is to find out the cultural needs of the plant and see where it is lacking. We probably have the plant on this site. Just do a search for the plant name.

This article will help:

https://www.gardeningknowhow.com/houseplants/hpgen/protecting-houseplants-from-cats.htm