There isn't enough info in your post to tell, really. Perhaps make a new post with more information, like how long have you had the tank? How big is it? How many fish? How often do you do water changes, and how much water to you change at a time? Did you add new fish lately? How much/how often do you feed them?
Probably the most common cause of fish death is too much ammonia in the tank. Ammonia comes from waste, like fish poop or old rotting fish food. There can be too much ammonia in a new tank when it is cycling, if there are too many fish, if they are fed too much, if their water is not changed often enough, or if a bunch of fish are added at once, etc. You can buy water testing kits to test the amount of ammonia in your water, or, you can also bring a sample of your tank water to a pet store, and a lot of them will test your water for free.