What is the law if the Kesuba became lost?
It is forbidden for a couple to remain even one hour without a Kesuba. Therefore if a couple lost the Kesuba they must have a new Kesuba written immediately, as soon as they are able. This Kesuba is called a Kesuba Deirkisa, or a lost Kesuba.
Marital relations: It is forbidden to have marital relations with one’s wife until the Kesuba is written [or collateral is given, as will be explained].
Yichud: It is disputed amongst Poskim whether Yichud is permitted with one’s wife until the Kesuba is written [or collateral is given, as will be explained].
Giving the wife collateral until the Kesuba is written: If one is unable to have a new Kesuba written for his wife right away, such as if the missing Kesuba was initially discovered on Shabbos, then he may give her collateral which amounts to the cost of the Kesuba, and take liability for any loss or decrease in value of the collateral. If this is done the couple may continue living together as normal and perform marital relations. As soon as the opportunity arrives, one must immediately have a new Kesuba written.
 Michaber 66/3
 Rama 66/2
 66/1 that this applies according to all
 The Michaber 66/1 prohibits Yichud, while the Rama 66/1 permits it, and so rules also Beis Shmuel 1/1 in name of Bach and Chekas Mechokek 66/1 [This dispute is recorded regarding a Kallah by her Chupa. It is unclear however if this dispute would also apply throughout the marriage, after the first relations of the couple. It is unclear if in such a case all Poskim would agree that Yichud is permitted, or if all Poskim would agree that Yichud is forbidden. [see Tiferes Yaakov 55/10]
 Michaber ibid
 Rama ibid