This is true, I used to have this problem with Cubase all the time on my old computer, I worked out that it happened when I had a lot of VSTis and VST fx open in the same session. My logic is that I was maxing out the ram and Cubase then wasn’t saving the sessions properly. After running 64 bit with more ram, I’ve never had this issue again but from now on you should turn on autosave and set it to 15min or less. That way you should have a working backup if it doesn’t save properly, or to be super safe just keep creating different file names and go through the alphabet, ie, filename-a, filename-b, … filename-z-a It’s handy anyway cause you can write what you did eg, filename-c_changedsnare
Permanent solution, run 64 bit and have more ram in your computer.
Solution for your song right now, if you have any sort of backup, open it and try to remember what you did after. If you have no backup, you have to re-record and/or reproduce and mix the song.