it has been on my window sill for about 2yrs and 3months...I'm not sure when it was repotted last!
If you've had this for two and a quarter years and never re-potted it the pot should be absolutely packed with roots if it was thriving. I suspect that the plant has not been doing terribly well for some time now. I have both Privet and Ficus and keep them outside, the Privet is outside year round and the Ficus just during summer.
Most bonsai will do better outside. Some can do well inside if provided the correct care but this is not always as easy as it might seem, especially for a casual grower. A very bright window, supplemental lighting, or both, are almost a necessity for indoor culture.
the soil feels abit slimy, I noticed today there is some white stuff on the soil.
The soil seems to be very dense/compacted/collapsed and would hold a lot of water. One possibility is that the soils is so dense that it excludes air and is wet for long periods of time. This can lead to a situation where the roots are dead/dying and can no longer take up enough water to support the foliage. What appears to be a lack of water (dry leaves) can actually be caused by watering too frequently. This is often made worse by attempting to revive the tree with, you guessed it, more water.
snowblind suggested re-potting it. While I am reluctant to recommend an ill timed re-potting in this case it may provide your best chance of resurrecting your tree. Unfortunately you are not likely to have any suitable potting medium on hand and the longer you wait the less likely a positive outcome becomes.
Trees roots need oxygen as much as water and the vast majority of bonsai enthusiasts use a mix that is very open and contains a high percentage of inorganic material of some sort.
Here are some threads for you to check out, read them over (follow the links as well) and get back to us with further questions. In the meantime hold off on watering it if the soil is damp.