Certified GKH Gardening Expert
This happens for many reasons. Have you changed its location lately? It the soil too wet or too dry? Fertilized recently? Have you checked for pests? This article will explain more: https://www.gardeningknowhow.com/garden-how-to/info/plant-dropping-leaves.htm
If it's become overwatered, I would also check the roots for root rot. It may be that a fungus has set in. Here is more information on root rot:
Schefflera - "umbrella tree" - are generally pretty tough customers when it comes to most stuff, like temperature, light being too low or high, being moved, drying out, etc. The thing they REALLY hate is wet roots. I suspect that's what's wrong with yours, because if it were some other stress factor, it would probably adapt. First, make sure it's not sitting in water. Second, test the soil to see how wet it is; you will need to investigate all the way to the bottom of the pot! If you have a relatively small plant - in a pot 10" diam or less - you can do this with a simple moisture meter from the plant store, or by digging soil up with a spoon, or by testing with a bamboo skewer-pretend you're testing a cake. If it's larger, you can test with a 1/2" wooden dowel. Scheffleras want to have their soil almost dry, all the way to the bottom of the pot, before they get watered again. If it's way too wet, it could take several weeks to dry. keep testing. Also, move the plant to a spot with more light, if possible.