Hello Sudheer, if the cards never been taken out and it has only stopped working after opening your PC, remove it using all the necessary anti-static precautions and clean the the contacts properly. Also check the slot for dirt.
If everything's clean and it still doesn't work, go through the usual process of trying to identify the problem by process of elimination e.g. try a different sound card, put the sound card into a different PC.
Also make sure you aren't turning your PC on and off too quickly, allow up to ten seconds for the power to discharge from the board before turning it back on.