half life 2 episode 1

hi i completed hal life 2 so i went on to episode one but the problem is that i cannot go through a wall which is blocked which is not meant to be blocked. I think there might be a problem woth my graphics card but it handled hal life 2 preety well with no problems . I have a radeon 9200Se graphocs card and i dnt know what the problwm is. The poitn where im stuck is after DOG throws you into a building through a car and Alex is following and there is a wall thats not there
I have tried changing the graphic settings still no luck..
Please help