Why the adjective word ```"frozen"``` after the verb in the sentence 
"**Mr. Dursley sat frozen in his armchair.**" in a book called 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 an adjective word. It can't after the verb sat.