To my knowledge Air France Flight 447 experienced stall. How was the stall detected and why wasn't the crew able 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.
What is a Stall?
A stall means that the flow of air over the wing is insufficient to produce enough lift to keep the aircraft in the air. This can be related to airspeed and angle of attack. Increase the angle of attack enough and the flow will separate from the airfoil.
Why not recovered?
From the investigation report carried out by the BEA, the pilots both reacted differently to the stall warning (which was false, frozen pitot tube). The aircraft entered a stall, which was perfectly correctable, and started falling down.