This question basically arose from https://islam.stackexchange.com/questions/47425/is-it-permissible-to-pause-salah-and-do-personal-work-and-continue-where-you-lef, which sounded to me a bit weird.  
There are a few reasons which allow a person to interrupt his fard (obligatory) prayer (while the matter seems a bit more easier for nafl/sunnah prayers), some of them have been quoted in the answer of my former question: https://islam.stackexchange.com/questions/27835/can-an-imam-interrupt-a-prayer-for-some-reason.

I just read a fatwa of sheikh al-Albani ([here][1]'s the link in Arabic) saying that there are two opinions on what one could do after the interruptions:

 - continue, for example you performed one raka'a and lost your tahara and made wudu' and came back to re join the congregation and count this full raka'a as achieved.
 - start from zero, assuming that the interrupted raka'a doesn't count as you've ended/interrupted your prayer.

I'd like to know which scholars or schools of thought hold the first opinion!


  [1]: https://www.sahab.net/forums/index.php?app=forums&module=forums&controller=topic&id=126334