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.
The ruling of the surah after Fatiha is mustahabb. There are no reports of any companions doing sajdah sahw due to forgetting a Surah. If you forget a Surah you continue. If you didn't recite 3 ayat in this rakat, then fine, because as I said, even if you recite 0 ayat it's perfectly fine.
But remember something important, if you do sajdah sahw before Salam for not reciting ayat, then unfortunately the prayer is invalid. That's why, you should do sajdah sahw in the cases narrated from our Prophet, sallAllahu 'alayhi wa sallam, or don't. (But if you wrongly do a sajdah sahw after salam, then this does not invalidate the prayer.)
So in summary: Reciting a Surah is mustahabb. Because it's mustahabb, there's no sajdah sahw for it. Hence if you do a sajdah sahw for it before Salam, you will need to make-up for the prayer even if it's a few days later that you remembered it, but if you did sajdah sahw for it after the salam, which you shouldn't have, the prayer isn't harmed.