Playing Half Life Game and Stuck!

I am playing half life and am stuck in the office level.

I have gone into the room with water, smashed the grate, crawled through the vent past the creatures and the fan and reached the other grate.

However, when I smash the grate and move forward, the floor caves in.

I land in a hallway, where shortly thereafter another part of the ceiling caves in and some of those “cute” little creatures fall through and come after me.

After dispensing with the creatures though, I don’t know how to progress.

I can see a short ladder up in the section of the ceiling where the creatures fell through and have tried to use the two boxes that are in the hallway to jump up onto the ladder but have been unable to do so.

Does someone know what I am suppose to do next? Am I suppose to get onto the ladder and enter what appears to be a vent opening right next to it in the ceiling? If so, how do I do it? If not, what am I suppose to do next? How do I move ahead?

Any help would really be appreciated.