Near the start of the level where the ship is sinking what do you do after you fight that first alien?
I've tried jumping up to the upside-down door way, but I don't seem to be able to climb into it and I can't climb the pile of boxes below it either. Is that the right direction? If so how do you get up there?

Sorry to necro this old post, but I'm having this exact same problem and unfortunately the video linked no longer exists. Does anyone know how to get past this glitch? It's not letting me jump up into the upside down doorway. All I can do is hang off the ledge!

Different video but same part, watch from 5:45. Just going to have to break the boxes and try jumping for the middle.
https://www.youtube.com/watch?v=goUXFAWCftE

Thanks - I got through it with a lot of button mashing! Just finished the game :)