@ Anusha : Dont worry, this is not a big deal.
Do one thing..... Take the identity card if you have and leave it if you dont have.
Draft a letter asking for the copy of the registration letter and take the signature of your boss.
I too lost my letter but by guidance of my boss , wrote a letter attaching the second letter from ICAI, i have received a letter from ICAI again.
Hope you will.
All the best.