I have made a schematic and layout in labcenter proteus but now unfortunately it fails to open. Can you please try opening it with your proteus software and if it succeeds please rename it and re-post here.
What I always do is whenever I have to make any changes to my current design, I create a copy of the file and save it on a backup folder. This helps me to keep a track on how I improved my design and also helps in roll back if the current file gets corrupted. I create a briefcase (in windows) of my working directory and save it to another place and do regular updates.