AF447 stall and failure to recover
The stall, for the most part, was not detected by the pilots: That was the entire problem.
There were many alarms and indications in the cockpit and that caused crew confusion, they neither knew which data to believe nor understood the situation that they were in. For much of the descent they could have recovered from the stall if they had been fully aware of the problem and had worked together. One pilot did try to recover the situation (by telling the other pilot to push the nose of the aircraft down) but he was not successful.

That answers the second part of your question too: The crew was able to recover from the stall but did not, due to their own confusion.