One tiny possibility: Look in your History panel and see if the initial snapshot it created when you opened the file is of any help. It will be listed right at the top.
Other than that, you're out of luck if you don't have a backup.
You may want to increase your History states setting. It won't help you now, but it might if you run into something similar in the future. It's in the Preferences - Performance dialog.
Another possible way to help avoid stuff like this is not flattening layered images themselves, but using Image > Duplicate with »Duplicate Merged Layers Only« checked and going from there.
If you saved as a JPG file you may find that the original PSD still exists - try looking on your OS for this.