My Question is "What To do if I forget surah in salaah?". Should I go to ruku or should I read any other surah instead of the surah I forgot. Few days back it happened to me and I just started reading another surah. Was I right?
That would be a Shaytan of the category of Khinzib (خنزب) who come and distract you in prayer, make you forget what you are reading etc. What you do is seek refuge in Allah, then blow three times on your left side. And when I say blow, it is between blowing and spitting, meaning you are not really bowing, and you are not really spitting, but in between that. Not many people practice this Sunnah now days, so it may come as a surprise to them. See the Hadith in Saheeh Muslim.
Lastly if you forget the Surah you were reading, you may change the surah or deal with the ayahs you already read, for perhaps in the second Rak'ah you may remember the rest of it, and Allah A'lam.
Recite it when you are next standing. If no more left, recite the missed sura when you are sitting after Et ta hayatu.
At the end of the salah before you salut the angels on both sides, perform 2 more sajdas for repentance for the mistake.