If the leaves of a Hadas have dried, the Hadas is invalid. If however it is not Halachicly considered dry, even though the leaves have withered, it still remains valid.
What is the definition of dry? If the leaves have dried to the point that they can be broken with ones nail and they have lost all their greenness, becoming white, it is Pasul. If the leaves have not lost their greenness, even if they look very dry and can be broken with a nail, they are still Kosher. [If they are dry and white but cannot be broken with a finger nail they are still valid.]
How many leaves must be dry to invalidate the Hadas? If the top leaves have remained completely fresh, then even if majority of the leaves have dried, they are still Kosher. However, if the top leaves have begun to whither and dry, even if they have not yet dried to the extent that invalidates them, some say it does not have the power to validate a Hadas whose majority of leaves are dry and hence the Hadas is invalid. Other opinions rule that even a withered top leaf validates a dry Hadas. Practically one should only rely on such a Hadas from the second day of Sukkos and onwards. If majority of the leaves [including the top] have dried to the point they can be broken with ones nail and have lost all their greenness, turning white, it is Pasul.
If only the top leaves are dry: If the top leaves of the Hadas have dried to the extent to make them Pasul some opinions rule the Hadas nevertheless remains valid. Others rule it is invalid, even if the rest of the leaves have remained fresh. Practically one should be stringent to use another Hadas if it is available. If another Hadas is not available then one may use it with a blessing.
On Chol Hamoed: Even if the top leaves have begun to dry, as long as they are not completely dry, the Hadas is Kosher even if majority of its other leaves have dried.
Second day of Diaspora: On the second day of Sukkos in the Diaspora if no other Hadas is available one may use a Hadas whose leaves have dried so long as its top is not completely dry. One is to use it without a Bracha.
If the top leaves have remained completely fresh, then even if majority of the leaves have dried, it is Kosher. However if the top leaves have begun to dry, even if they have not yet dried to the extent that invalidates them, some say it does not have the power to validate a Hadas whose leaves are majority dry. Practically one should only rely on such a Hadas from the second day of Sukkos and onwards.
Top leaves: If the top [leaves with the stem] was cut off or the top leaves dried up [see above for definition] one should exchange the Hadas.
What is the law if the leaves of a Hadas are drooping downwards?
This matter is disputed in Poskim.
The law of cracked or cut leaves?
If only minority of the Hadas contains split leaves it is valid. If majority contains split leaves it is disputed whether it is valid.
 646/8; As this is not considered Hadar. [ibid]
The reason: Because one is still able to insert the Hadas into water and have its healthy state. [ibid]
 The reason: The reason for this is because the main beauty of the Hadas is the top which is what one sees at first sight and thus so long as the top of the Hadas remains dry it is considered Hadar and is valid. [ibid]
 This is despite the fact that if the entire Hadas was withered it is valid. [ibid]
 The reason: Now although whenever the head of one of the other Minim have been chopped off the species is invalid nevertheless by a Hadas it remains valid. The reason for this is because the leaves of a Hadas cover over the top branch that was cut off and it is hence not noticeable that the Hadas has lost its head and its Hadar is thus not readily effected. [ibid]
 The reason: As since the head of the Hadas is not Hadar the Hadas is invalid. [ibid]
 Piskeiy Teshuvos 646/2
 Piskeiy Teshuvos 646/2