HI,
try pressing (shit+C) see what happens. Please check that you have not by accident moved from colour to transparent or alpha mode. Maybe try shif + option + C.
If this does not work, you can refer to the backup files that apple motion creates sort of saves versions of the same file. look in your home directory/documents/apple motion/ then find the file you have worked on. Maybe you can recover an earlier version of your project. This could mean that you have to redo some part of your project but it would surely beat recreating the complete projects.