One should not interrupt his Amida prayer, even if the king of Israel asks him how he is doing, he should not respond. In today's times, Hakham Yishak Yosef states that this even applies to a non-Jewish leader, since they don't have the power to put one to death for not replying hello.
One is ONLY allowed to break his Amida prayer for something that will pose DANGER to his life--i.e. if a wild animal is running towards the person praying. If there is an imminent danger, even speaking is allowed if necessary- since this is a case of פקוח נפש (saving a life). In the case of stopping for danger, if one stopped during the first 3 berachot, then one must return to the beginning. If one stopped during the last 3 berachot, then return to רצה. If one stopped for enough time during which one could finish a complete Amida, then he/she must start over.
(Shulhan Arukh and Yalkut Yosef [Kissur Shulhan Arukh] Orah Hayyim Siman 104)
One is ONLY allowed to break his Amida prayer for something that will pose DANGER to his life--i.e. if a wild animal is running towards the person praying. If there is an imminent danger, even speaking is allowed if necessary- since this is a case of פקוח נפש (saving a life). In the case of stopping for danger, if one stopped during the first 3 berachot, then one must return to the beginning. If one stopped during the last 3 berachot, then return to רצה. If one stopped for enough time during which one could finish a complete Amida, then he/she must start over.
(Shulhan Arukh and Yalkut Yosef [Kissur Shulhan Arukh] Orah Hayyim Siman 104)