I have a PC ranger in my campaign whose alignment just changed to CN from CG. I give the usual 3 strikes and then I change a players alignment. As a ranger though does she lose all the abilities that she has gained?
Absolutely nothing. The ranger class has no alignment requirements and no aligned class features. There is no such thing as an ex-ranger; they cannot “fall.” They aren’t even banned from using spells of conflicting alignments, the way clerics are.1 Using aligned spells is an aligned act, but in most cases a minor one and would only shift alignment over prolonged and frequent use.
Also, there is no such thing as a “usual 3 strikes” unless that is something you have established for your games. I would strongly recommend reconsidering such a system; issues of alignment are too complicated and messy for that. I also strongly suggest that you avoid considering alignment change as a punishment, which appears to be what you are doing here. If a character consistently acts a particular alignment for a prolonged period of time, that would be a good time to talk to the player about the character and his or her alignment.
1 This is debatable; the Spells class feature says “a ranger prepares and casts spells the way a cleric does,” which I take to mean it works like the cleric’s Spells class feature; the cleric’s restriction on aligned spells is listed separately in the Chaotic, Evil, Good, and Lawful Spells class feature. Thus, I consider that section not to be included in the ranger’s Spells class feature. This seems consistent with their description, e.g.
Religion: Though a ranger gains his divine spells from the power of nature, he like anyone else may worship a chosen deity.
from the Player’s Handbook, page 46. The statement from Chaotic, Evil, Good, and Lawful Spells about being banned from spells that conflict with a deity’s alignment therefore seems inconsistent, and it also seems out of keeping for nature to impose alignment requirements.
But ultimately, you could argue it the other way; it is at least ambiguous. In that case, the ranger going from CG to CN would simply mean he or she can now choose to use Evil spells, which were previously not allowed.