LOL!!!!
You said you finished that before? I know that prob, it happend to me too.
If I'm correct, you finished the level by taking the elevator up. But you'd forget the flint clock. Once the lift is up, it will stay up. Even if you re-enter the hub. Your answer is:
Open a previously saved game (BEFORE the lift-accident).
Avoid the scrab, find a way up to the flint clock.
Take the lift up.
Go through the door.
Finish the game.
It should work. However, I didn't played it for a long time. Somethings might be incorrect (except for the lift-thing). Believe me, I have a good memory.
BTW, good luck!!