That's weird. Did you enter the first arena room, the one after a long vertical corridor you fall into? (did the music change and did you notice enemies spawning in waves?)
If not, either you got a bug or you didn't find the path to go there!
I gave it another go, turns out that after reversing for the first time I just had to fall to continue with the game!
I guess it wasnt very clear to me haha!
