Sounds so weired, the key frames are not there but the animation is! by the way if you have a previous version of your work which you might save like the day before (when the animation of the camera was already done properly), open it and check whether the problem is also there or not, even you may check an older and older versions. if the file was OK, then just simply select all the key frames you want from the camera portion of that file and then copy and paste it in your new file then remember to “save as” in a new file again.
Well, I think if the new file which you will create got the same problem the very next time you open it again, it might be a bug or something from your AE or even your OS! Or perhaps you are using an effect which the key frames are lie under one of the portions of that certain effect!
Hope this help.
Good luck!
Shahin