I'm sorry this has happened but I don't think anyone is going to know the answer without lots more information and troubleshooting.
The best option to recover your original edits is if you have PD's Autosave turned on. Go to Preferences | Editing and see if the Auto save box is checked as shown below:
If it is, you can open one of the recent temprary projects from the folder specified in the Browse window and that should hopefully let you work on a copy of the project from before the masking incident occurred.
If autosave was not enabled, the only thing you can do is try undoing the last 50 edits to see if one of them caused the issue.