Q.My Plumbagos Have Lost Their Leaves Overnight. What Insects Or Bugs Should I Look For, Any Idea?
What type of insecticide could I use?

Certified GKH Gardening Expert
Unfortunately, your photos did not come through.
It can be very difficult to pin down the pest without seeing them, but it sounds like caterpillars may be to blame.
Here is an article that will help:
https://www.gardeningknowhow.com/plant-problems/pests/insects/prevent-caterpillars.htm
This article will help you with the care of Plumbago:
https://www.gardeningknowhow.com/ornamental/shrubs/plumbago/growing-plumbago-plants.htm