I originally posted this on the SS page about a week ago but since no-one has given an answer, I thought maybe not enough people read that forum. Anyone got a fix for this? I'd really like to be able to finish the campaign.
http://www.gog.com/forum/silent_storm/freezing_on_a_certain_mission_sentinels_spoilers

This has happened to me on various other occasions, but not on this game in particular. I would give you the advice of maybe uninstalling/re-installing it, and when it asks if you want to keep your saved game, then of course say yes. If it still has problems, then contact the makers of the game, hopefully they can get back to you with a solution.

I am a bit of a dummy when it comes to computer technical problems, so this is pretty much the best thing I can think of. :) I hope this does help!