Can't climb ladder below railway bridge in HL2 on X Box 360.

I'm playing Half Life 2 on X box 360 (Orange Box) and I'm at the railway bridge that's blocked with an energy field. I've crossed underneath the bridge via the supports,pressed the switch at the far end to de-activate the force field,and shot down the gunship. When I go back across the underside of the bridge to get back to the car on the cliff top, it wont let me climb the ladder that goes from the metal balcony to the top of the bridge support where the door is. I've also found that if I try to descend the ladder before getting onto the bridge it won't let me. This of course means that having crossed the bridge,de-activated the force field and shot down the gunship, I can't go any further because I can't get off the bridge. I bought a second copy of the game thinking it might be a disk error but the problem is still there.Can anyone help please.