Sounds like its corrupt ... do u recall having any problems or error messages last time you worked on it? In any case, try opening an new AE project and importing that one into it instead of opening it directly. You can also try, opening it either way with all plugins, etc. disabled.
Of course the best option would be to just use your backup copy of the project, but my guess is that you wouldn't be posting if you had one.
If none of those work, search the forum for other things to try ... has been discussed many many times.