Certified GKH Gardening Expert
Unless you can see signs of droppings, footprints, insect parts, or other signs, you may not be able to find the exact culprit.
If you can find evidence of whatever it is, then you can snap a few pictures and send them in here. Then we can begin to find out what it is that has destroyed your basil.
In the meantime, here is a collection of articles that will help you to craft your own pest control: https://www.gardeningknowhow.com/special/organic