If a person began reciting "Ata Honen" in the Amida on Shabbat instead of the text for Shabbat, he should complete the Beracha and then recite the proper text. For example, if during Arbit on Friday night one began reciting "Ata Honen" instead of "Ata Kidashta," he should complete the Beracha and recite through, "Baruch Ata Hashem Honen Ha’da’at," and then begin "Ata Kidashta." This applies as well to the other Shabbat Amida prayers. The reason for this Halacha is because the weekday Amida is fundamentally appropriate even on Shabbat. The Rabbis established other, shorter prayer texts for the Amida on Shabbat in order not to overburden us with a lengthy service, but in principle, the weekday Amida can be recited even on Shabbat. Therefore, one who mistakenly began reciting the weekday text should complete the Beracha, as it would be disrespectful to stop in the middle of a Beracha.
If a person forgot this Halacha and stopped in the middle of "Ata Honen," and he realized his mistake only after he recited the Beracha of "Mekadesh Ha’Shabbat," he does not go back to complete the Beracha of "Ata Honen." Although he should have completed the Beracha, once he recites "Mekadesh Ha’Shabbat" he is no longer able to complete the Beracha of "Ata Honen." This is the ruling of the Hid"a (Rav Haim Yosef David Azulai, 1724-1807), in his Mahazik Beracha.
If a person recited the entire weekday Amida on Shabbat, he has not fulfilled his obligation, and he must then recite the proper Amida for Shabbat. Despite the fact that, as mentioned, the weekday Amida is fundamentally suitable even for Shabbat, nevertheless, one must make mention of Shabbat at some point in the Amida. Therefore, one who recites the normal weekday Amiida on Shabbat has not fulfilled his obligation. This is the ruling of the Shulhan Aruch (Orah Haim 268:4). However, if one recited the weekday Amida but made some mention of Shabbat – for example, if he recited, "Reseh Na Bi’mnuhatenu" in the Beracha of "Shema Kolenu" – then he does not have to recite a new Amida. Since he mentioned Shabbat somewhere in the Amida, he has fulfilled his obligation, even though he recited a weekday Amida.
Summary: If somebody began reciting "Ata Honen" in the Amida on Shabbat, he should complete the Beracha and then proceed to the proper text for Shabbat. If one recited the entire weekday Amida on Shabbat, he must then recite the Amida for Shabbat.