Why the adjective word "frozen" after the verb in the sentence "Mr. Dursley sat frozen in his armchair." in Harry Poter <Harry Potter and the Sorcerer's Stone>.

I change the sentence. "Mr. Dursley sat happy in his armchair." The sentence is wrong. "happy" and "frozen" are adjective words. Why I can't replace "frozen" with "happy".

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