We are beginning siman 26, where the Chayei Adam discusses one who forgets to make kiddush. The Chayei Adam begins with a case of a person who forgot to make kiddush, washed, made the bracha for hamotzi, and then remembered they did not make kiddush–before they ate from the hamotzi. We know that a person can make kiddush on hamotzi if they do not have wine. The regular procedure would be to say the bracha of hamotzi, followed by the bracha of mekadeish hashabbos. In our case, the person is technically in the perfect spot to continue kiddush and recite the bracha of mekadeish hashabbos. The only potential problem is that the person did not have the intent to recite kiddush when he recited the bracha of hamotzi. Nevertheless, the Chayei Adam writes it is not a problem, and he should continue the kiddush over lechem mishneh, meaning, if he has already cut the bread, he should bring another, whole, bread, and continue with the next bracha of kiddush.
He would not say veyechulu at that point, but sometime during the meal he should say veyechula, preferably over a cup of wine.
In this case, even if the wine is right in front of the person, they should continue with kiddush over the bread rather than switch to wine, because at this point (after having inadvertently made the bracha of hamotzi), it would be a hefsek to make the bracha of borei pri hagafen. Although it is not a hefsek to make kiddush between al netilas yadayim and hamotzi, it is a hefsek between the bracha of hamotzi and eating the bread, since it is possible to make kiddush over the bread.
If the person already said hamotzi and ate from the bread, they should make kiddush over wine, and recite from the beginning of kiddush. If they do not have wine, they should make the full kiddush while holding bread (i.e., beginning from vayechulu), but skip the bracha of borei pri hagafen. According to what we mentioned above, it would be preferable to hold lechem mishneh when making kiddush.
Similarly, if one made a borei pri hagafen on wine before their meal, without realizing they need to make kiddush, they recite the full kiddush but skip the bracha of borei pri hagafen.
If a person finishes their meal and forgot to say kiddush, they recite kiddush at whatever point they remember to make kiddush. However, they would have to make sure to eat something after kiddush, to ensure they fulfill the chiyuv of kiddush b’mokom seudah.
The Chayei Adam does not discuss one who completely forgot to make kiddush on Friday Night. The halacha is that they would recite the bracha of hagafen and the bracha of mekadeish hashabbos (skipping the pesukim of vayechulu), and they would not recite any of the pesukim normally recited on Shabbos day.
Summary
Regarding a person who forgot to make kiddush:
If they remember between the bracha of hamotzi and eating bread: they continue reciting kiddush from that point.
If they remember after eating bread: they recite the full kiddush on wine when they remember
If they only have bread, they skip the bracha of hamotzi in their kiddush
If they remember after having drunk wine: they recite the full kiddush and skip the bracha of hagafen
If they remember after finishing their meal: they recite kiddush whenever they remember and eat something to fulfill kiddush b’mokom seudah
If they remember the next morning, they recite the Friday Night kiddush instead of the Shabbos Day kiddush.
