Why is the adjective word "frozen" after the verb in *Harry Potter and the Sorcerer's Stone*?

> Mr. Dursley sat frozen in his armchair.

If I change the sentence to

> Mr. Dursley sat happy in his armchair.

The sentence is wrong, because "happy and "frozen" are adjective words. Why can't I replace "frozen" with "happy"?

In my opinion, the "frozen" is an adjective word. It can't go after the verb "sat".