What should I do if I start reciting a surah during an obligatory prayer and forget the rest of it; should I bow, or should I recite another surah?

If you forget an verse during prayer—other than Al-Fatihah—and are unable to recall it, and no one prompts you, then you have the choice to either bow (perform ruku'), or move on to recite another verse or another surah. Indeed, the Prophet, peace and blessings be upon him, was once confused in his prayer and no one prompted him. It is incumbent upon the congregant not to hasten to prompt, and upon the Imam not to force them into it; rather, he should bow when the time for bowing comes, or move on to another verse.