There are a couple of reasons this can happen.

First of all, from the campaign mission loading screen, make sure that you select the CONTINUE CAMPAIGN button and not the PLAY MISSION button. You'd be surprised how many people have done that in error!

If that is not the problem, you'll need to delete all your quick saves from the following folder. The quicksave files are the ones named quicksave_*.dsl. do not delete any other files.

GRAW 2 Installation > settings > profiles > (yourprofile) > savegames

If that does not work, you can install the GRAW 2 Complete pack of mission saves. Note that this will mean you will continue with a different set of Ghosts, and is really a last gasp solution.

NB The above was written when GRAW 2 was at Version 1.02 in July 2007. Subsequent patches will probably resolve this issue.