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 [or an invalidation was found in it then] 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 engage in intimacy 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 new Kesuba is written [or collateral is given, as will be explained]. [Practically the Ashkenazi custom is to be lenient to permit Yichud.]
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 [i.e. cash, or objects] which amounts to the value 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 engage in marital relations. As soon as the opportunity arrives, one must immediately have a new Kesuba written.
Making a Kinyan in front of witnesses: If one is unable to have a new Kesuba written for his wife right away then another alternative to giving collateral to the wife, is for the husband to obligate himself with a Kinyan towards the Kesuba sum in front of two kosher witnesses. If this is done, the couple may continue living together as normal and engage in intimacy. Nonetheless, as soon as the opportunity arrives, one must immediately have a new Kesuba written.
What is the law if one knows that the Kesuba is somewhere in the house but simply cannot find it due to being seemingly misplaced, or due to him forgetting where it was put?
The above law only applies if one knows that the Kesuba has become lost or destroyed, such as if G-d forbid a fire burned down the home, or one searched through all of his belongings and cannot find it and there is no other area that it could’ve been misplaced. If, however, it is somewhere in the house and he just does not know where, then a new Kesuba is not required to be written and the couple may resume our life as normal.
 See Chelkas Yaakov E.H. 90; Tzitzi Eliezer 11:67; 19:39; Nitei Gavriel Nissuin 1:33; Kesuba Kehilchasa 2:9-10
 Michaber E.H. 66:3; Kesubos 39b and 54a; Yevamos 89a; Bava Kama 89a
 Nitei Gavriel 33:1
 Michaber E.H. 66:3 and Rama ibid that so is the custom today and one may not swerve from it even though from the letter of the law there is no requirement to write a new Kesuba being that in any event one cannot divorce his wife against her will; Chelkas Mechokeik 66:17 and other Achronim however argue that it is required from the letter of the law; See Tzitz Eliezer 19:39
Relying on Beis Din copy of Kesuba: It is disputed amongst the Poskim as to whether one may rely on a copy of the Kesuba that is found in the offices of the Rabbinate, as is the common practice here in Israel for the Rabbanut to keep a copy of all Kesubos. Some Poskim rule that it is valid to temporarily permit intimacy [Shemesh Umagein 62; Rav Ovadia Yosef in Chuppah Kehalacha 6 footnote 10 and Mayan Omer 9:266; Nesuin Kehilchasam 11:225; Pischeiy Chosehn 8; Shalmei Simcha 34 p. 253 in name of Rav SZ”A; Poskim in Nitei Gavriel 33:6 footnote 10], while other Poskim rule that it is invalid being that it is a mere copy of the signatures and cannot be used to collect in court. [Teshuvos Vehanhagos 1:760; Or Letziyon Kesubos 121; Koveitz Beis Halevi 7:60; Kesuba Kehilchasa p. 307 in name of Rav Elyashiv]
 Michaber E.H. 66:1 [this applies according to all]
 The Michaber 66:1 [and Rambam Ishus 10:7] prohibits Yichud, while the Rama 66:1 permits it, and so rules also Beis Shmuel 1:1 in name of Bach and Chelkas 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]
 Heard from Rav Y. Gurary, Chief Rabbi of Chulon
 Michaber E.H. 66:2; Kesubos 7a
 Nitei Gavriel 33:3
May a check or promissory note be written? Seemingly a check is also valid as it is no different than the Kesuba which is itself a promissory note. [Kesuba Kehilchasa p. 8; Koveitz Mibeis Levi 7 Kesubos 3:3]
 Michaber ibid regarding Shabbos when cash cannot be given
 Michaber ibid
 Rama ibid
 Michaber 66:1 regarding all times that this is a valid alternative to writing an actual Kesuba, and Rama ibid that this method may be used in a time of need; Rambam Ishus 10:39; Minhagei Mitzrayim E.H. 15; Radbaz 4:174; However, see Bach 66 who invalidates relying on witnesses
 Vetzaruch Iyun if according to the Rama he may do this alternative even if he has collateral available to give, as the Michaber 66:1 initially invalidates the giving of collateral while validates the option of the new Kinyan even initially, which perhaps implies that according to the Rama one can choose to do this option of a Kinyan even if he has collateral available, and the reason why only the option of collateral was mentioned in 66:2 is because it is referring to a case that it occurred on Shabbos. Vetzaruch Iyun.
 Rama ibid
 Shevet Halevi 8:288 based on C.M. 41:3; Nitei Gavriel 33:4