Yesterday,when I left my plastic chair in front of com and touched my friends face from back side,both we feel and heard an electric shock.even a white light also spearked. I wear a silver chain with rudraksha,and a silver-gold-iron-copper combo bangle.
Will you please help me to collect logic behind this?

I would assume that you received a static electric discharge. I don't think your silver chain had anything to do with it, but the plastic chair and what you were wearing probably did.

Thank you perito,
I wear that bangle on my right hand.and the spark happened on my left hand.
Please analise this more to me sir.
I want to know the whole system by deatil.

Static electric charges can accumulate in the body. However, the situation is, physically, unstable since there is a separation of charge. If you touch a "grounded" person or anything that has a lower charge than you do, the electricity will discharge. You see (and often hear) a spark. If the spark is close to your eye, you can see a flash.

Electrons are acquired often by rubbing synthetic material on plastic. (It's not exclusive to these materials, but they seem to be more likely to produce static electric charges than anything else). It's the same phenomenon that I did when I was a kid - rubbing rubber-soled shoes on a synthetic carpet and then touching my little sister's ear to see her jump. The body, itself, is filled with water and salts. Electricity is conducted very easily through the body. The electric charge will move to wherever it must to discharge.

If the air is humid, the electricity will often dissipate through the air so that you don't even notice it. That's why most static discharges of this sort occur only in very dry air -- often in heated buildings during the winter.

The bangle has nothing to do with it.

Here's a Wikipedia article that will explain in more detail:

http://en.wikipedia.org/wiki/Static_electricity