Let me chime in to say I have the same problem, so it's not specific to the prior poster. It's very frustrating, as I've made some pretty time-consuming mixes with cross-fades, etc.
I do find that if I go into each track and select "edit audio," the prior edits are still there, so if I hit "done" they will return. But I have to go through and do that for each song on the project. Major pain. Never had this issue before. (I'll also add that, just as this problem cropped up, my projects are no longer saving cd text, even though that option is selected. Not sure if that is a related issue.)