I've had a Bonsai tree for a few months without any problems-then after watrering it last night---the leaves have turned yellow and are falling out!

Sounds like you over watered it, it may have been sitting in water all night. Take paper towels and push the soil aside enough to get them down into the pot...you can twist the towels into a cone shape. Turn the pot so it's elevated in one side so the water drains toward the paper towels and let the towels soak up as much water as possible. Keep changing the towels when saturated. Hopefully, you caught it in time. Good luck.