Question

when we block our nose why taste comes different

Answer 1

Flavor = Taste + Smell

What we call "Flavor" is a combination of taste and smell.

There are only five taste: salty sweet, sour, bitter and savory/umami.  Compared to taste, there are virtually unlimited number of scents humans can detect.

So how many? It's one trillion according to The Science magazine:
"...roughly 1 trillion scents that the human nose and brain are capable of distinguishing from each other..."

Therefore, when your nose is blocked, basically you can only "taste" the food and unable to "smell", missing the majority of your food's flavor. This is the reason why you were unable to taste anything when you had a cold.

Answer 2

When you can't smell, you can still taste, but what you are used to describing as 'taste' is actually the combined senses from your nose and tongue and other parts of your mouth. A famous experiment with smell and taste has blindfolded, nose blocked testers taste apples and onions and are unable to tell the difference.