Dear Rabbi Simon
If you’re in the middle of the Amida (or Shema) and suddenly feel ill and need to stop or use the facilities, what do you do upon restarting – go back to the beginning or pick up from where you left off?
Got to Go
TY for your Q. This is actually a bit complicated, as the answer depends on a number of variables, and may be different in the Amidah vs Shema. The rule of thumb is that if the interruption is of a duration longer than it takes one (ie the particular individual) to recite the entire Amidah, or the Shema and its blessings (according to the circumstances), and the interruption was not by choice but by halakhic force of circumstances, eg ineluctable call of nature (as in your question), one must start over at the beginning. However if the interruption was of shorter duration, or not actually dictated by halakhic necessity, s/he should just go back to the beginning of the blessing or paragraph preceding the interruption.
For further research, see Shulkhan Arukh OH 65:1 and 104:5 and Mishneh Berurahthere.
I hope this is helpful (at least partially).
Rabbi Rashi Simon