As the word implies, the Amida must be recited while standing, but there are situations when this is not practical. The Shulhan Aruch (O.H. 94:4) writes that if by standing to pray one will not have proper concentration, one may sit down to do so. One contemporary example would be if one would were anxious due to the normal jostling of a subway and would concentrate better if one prayed while sitting down. The Shulhan Aruch goes on to say that there is a strict opinion that requires one to stand at least for the first blessing of the Amida and says that one should follow this opinion if it is safe to do so.
The Shulhan Aruch (ibid:9) says that if one had to pray sitting down out of necessity, one should repeat the prayer standing up when it becomes possible to do so. Typically, when a prayer needs to be repeated or was missed, one must make a stipulation that if the prayer is truly required then it should count as a prayer, and if it is not truly required then it should be considered as a voluntary prayer. Interestingly, in this case the Shulhan Aruch says that one need not make the stipulation. In other words, standing up for a prayer is so important that one should repeat is as though one did not pray at all. Nevertheless, the HIDA (Birke Yosef, § 94:5) and Rabbi Yehuda Ayash (Matte Yehuda, § 94) write that the custom is not like the Shulhan Aruch and one need not repeat one’s prayer. Rabbi Baruch Toledano (Kitzur Shulhan Aruch), quoting the Kaf HaHaim (O.H. 94), and Rabbi Ovadia Yosef (Yabia Omer, vol. III, § 9) concur. Rabbi David Yosef (Halacha Berura, Otzrot Haim, pg. 50), based on a modified ruling by his father Rav Ovadia, and Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 45, § 26) both write that one should repeat one’s prayer and make a stipulation. Nonetheless, it appears that practically speaking, the custom is to be lenient and not repeat the Amida.
This being said, the importance of standing during the Amida should not be understated. If one has an opportunity to stand, even if it means pulling over and getting out of one’s car, one should not rely on the leniency to sit. Evidently, if it is not safe or there are other factors, one may sit
Summary: In extenuating circumstances, one may sit down to recite the Amida. In such a case, one need not repeat the Amida while standing up.