A betta laying on bottom of tank can be nothing to worry about. However, if your betta fish is not eating and also laying at the bottom, there could be a problem. Changes in the water temperature or chemistry can often cause this behavior, and the most common issue is ammonia poisoning.
How many years does a betta fish live?
Why is my betta fish not moving but still breathing?
It Might Be Resting or Sleeping
Betta fish are naturally very lively fish, but just like us, they need a period of rest to allow themselves to recharge. Some fish hover in the water while they sleep, so an unmoving betta may simply be resting or sleeping.