I would say обсуждение or тема.
Топик is the English word, however there is a word топика used as a scientific term (meaning complex of themes).

In this case I would say: Мою тему сорвали. Обсуждение, начатое мной, сорвали. Тему, начатую мной, сорвали.

There is the expression "сходить с рельсов". It's possible to say something like Тема сошла с рельсов. - which sounds here more figurative than the English 'derail' (in English it's a set idiom, in Russian it sounds OK, but not so typical for such situations).