when we have a cold why we can not smell?

The swelling or mucus in your nose keeps the air which carries odours from reaching the nerves that detect and register "smells" in your brain. In case of a cold, the nasal passages are swollen shut and you won't be able to smell or taste anything. When the infection or cold starts to subside, the sense of smell is now too strong due to nerves becoming irritated. The nerve ending that smell and taste are renewed after about every 40 days.

Hope this answer will clear your doubts
Regards
Akanksha Arora