Check that the material you have assigned is the issue. Change it to a
different material in sketchup and see if it works.
Other issues may be opacity setting? It could be as simple as hitting
"2-sided" in crysis material editor.
If you can post more info such as do you have several different
materials in your model? Are you checking the "reverse faces" in
sketchup? (do this by clicking that green surface icon in the playup
toolbar). Could you post a picture of what it looks like in sketchup
vs. crysis.
Cheers