Sotah chapter 4, mishnah 2. This mishnah lists women who cannot drink the sotah water, and since they themselves are the reason they do not drink, they also do not collect their kesubah. They remain forbidden to their husbands, since there is no way to resolve the doubt through drinking the water, and they do not receive the kesubah because the obstacle comes from them.
"Ve'elu lo shosos velo noteles kesubah":
These are the women who cannot drink, so that there is no way to determine whether or not they became defiled, and who also do not receive the kesubah:
"Ha'omeres teme'ah ani" - a woman who admits that she was defiled. Once she has admitted it, there is no longer any doubt here, there is no place for giving her the sotah water to drink, and she does not receive her kesubah.
"Sheba'u lah edim shenitme'ah" - witnesses came and testified that she committed adultery while she was in seclusion. It is worth noting that even if she were to drink, that would not contradict the witnesses; their testimony stands in full force, and therefore she neither drinks nor collects a kesubah.
"Veha'omeres eini shosah" - a woman who refuses to drink. Once she has decided on her own not to drink, she becomes forbidden to her husband and does not receive a kesubah.
The cases in which she collects a kesubah and does not drink:
By contrast, when the obstacle to drinking comes from the husband's side, she is not made to forfeit her kesubah:
"Amar ba'alah eini mashkeh" - the husband refuses to have her drink and takes no part in the process, and thereby prevents it from taking place.
"Umi sheba aleha baderech" - the husband lived with her on their way to the Beis HaMikdash, at a time when she is forbidden to him until her innocence is established.
In these two cases the obstacle is his fault, and therefore "notlos kesubah velo shosos" - she does not drink, since the process cannot be carried out, but she does receive her kesubah.
If the husband died before she drank:
What is the law regarding a woman who was supposed to drink and whose husband died? The husband's presence is required for the drinking, and once he has died there is no longer any purpose in drinking, except for one possible purpose: to determine whether or not she is entitled to her kesubah. On this question Beis Shammai and Beis Hillel disagreed:
Beis Shammai say: "notlos kesubah velo shosos" - she does not drink, for the verse states "vehevi ha'ish es ishto" - "and the man shall bring his wife," and the Torah made the drinking dependent on the husband bringing her; since he died, there is no place for the drinking. But she does collect her kesubah. The Gemara explains their reasoning: Beis Shammai hold "shtar ha'omed ligbos kegavui dami" - any document that stands to be collected, even though it has not yet been collected, is viewed as though it were already collected. It emerges that she is like the owner of the kesubah money, and when a doubt arises as to whether she is entitled to the kesubah, we decide in her favor, since it is as though she had already collected it.
Beis Hillel say: "lo shosos velo notlos kesubah" - they too agree that she does not drink, for the same reason, but they disagree with this principle and hold that a document standing to be collected is not considered collected. A document that stands to be collected but has not yet been collected is not viewed as collected, and therefore the burden is on her to bring proof that the kesubah is due to her, and the heirs get to hold on to their money.
And this is the rule known in the language of the Mishnah as "hamotzi mechavero alav hare'ayah": one who seeks to extract money from another is the one who must bring proof. In our case, according to Beis Hillel, the woman is the one seeking to extract, and therefore the burden of proof is on her.