If it is within the 18 minutes before sundown, she can light. (Not sure if this is true for the entire 18 minutes, since tosfos Shabbos is required both at the beginning and the end of Shabbos; I remember reading an opinion here that 4 minutes before sunset is the latest. On the other hand, I have also read that tosfos Shabbos is considered when calculating the zmanim, so by just following the prescribed times, one already includes tosfos Shabbos. Various opinions are discussed in comments here. Before 4 minutes, she can definitely still light.)
She definitely cannot light at or after sunset--that is, during Shabbos--and I don't think there is any reason she would be justified in following Rabbeinu Tam for one week, unless that were her regular minhag.
According to several canonical sources (cited here), a woman must add an extra candle going forward for each week that she misses lighting as a result of her own negligence. This discussion suggests that this k'nas (penalty) applies to single men, too. (According to Dov Eisenberg's Guide for the Jewish Woman and Girl, it doesn't apply to baalos teshuva. [Converts are not mentioned but one would think according to this it wouldn't apply to them kal v'chomer.])
I had never heard that she could ask a non-Jew to light for her if missing candle-lighting caused her distress, and have never heard of anyone's doing that. But I assume it is a legitimate possibility based on Fred's source (Bei'ur Halacha, p. 261). I would also wonder if her husband could light for her if he had not yet davened.
Some women have the ~custom to make a blessing in front of candles lit by others if they miss lighting (or generally do not light candles--as is the case for many Yeshivish single women.) This should be, if anything, a blessing without Hashem's name (source: https://www.ou.org/torah/halacha/hashoneh-halachos/1097-mikvah-friday-night/)
Edited to Add: One thing she can do, and which people frequently do, if she expects to miss candle-lighting is ask another woman--anywhere in the world--to have her in mind while she lights. If she has already missed, she may be consoled to hear that someone else had her in mind. (Some people deliberately have a lot of others in mind in case they should miss.) Note that none of this means that she was actually yotzei the mitzvah.
I don't want to overstate how common this is, though--most frum women take hadlakat nerot very seriously and would only miss it in an inevitable circumstance.