If one needed to relieve himself during davening – SE, and the time he spent in the bathroom was longer than the time it takes him to say SE, now he needs to start over. If when he exits the restroom, they are saying Kedusha, may he answer, or do we say that since he has to daven again he considered to be in the middle of SE? May he repeat/ start over his SE later, when the next minyan will be available?
If he already had reached Elokai Netzor, then does he need to start over or is he yotzi since he basically said everything?
The question is a very interesting one. After consulting numerous talmidei chachmim, it seems that it would be considered like he is still in middle of SE and he may not answer kaddish, etc., and he has to daven right away without waiting or talking out. Even though the first SE was not completed, it is not considered a hefsek, that now he can do what he wants and just daven again later.
If he already finished the main part of SE, and he is Elokai Netzor he doesn’t repeat SE.
See Eretz tzvi 2-7, Shaul U’mashiv first edition 3-169, they both talk about f he forgot Yaaleh V’yavo etc. if he may answer kiddusha, and they both say that he may not answer. This case is even worse, as here he surely was not yotza SE the first time since he didn’t even finish it. R’ Y. Cahen, R’ Y.A. Dunner, Shlit”a, both said that it is like he is between geula l’tefilah and he can’t answer.R’ S. Felder shlit”a went even further that even though when a person is in middle of SE and they are up to Kidusha he may listen to it and be part of it with listening, but here it is like he is between Geula l’tefilah and he may not even do that! See Sharei Teshuva O”CH 66-13.