Hi Chandan it is very common problem when the dogs are getting old and lost their sight. It is just because of the thin layer is covered over your dogs eye and your dogs sight become blur after some time it is totally lost their viewing sense. Frist of all you can go to your vat and ask what to do, if he said that he operates and everything will fine than its good but if not than follow some instruction:-
1. Do not change the interior of your house or dogs house.
2. Insure that things around the dog (inside your house) is at a level which does not hamper the mobility of the dog
3. Insure that somebody always stay at home to take care of them.
4. Avoids use of stairs when you are with your dog because its panic for your older one.
And be patience its your love one. He is the one which give you the several memorable moments and now its the time to pay you back with your love and cure.