If you are using time machine it does a backup every couple hours, so you can get previous versions that way.
As a habit I save my file with a different name ever couple hours, just in case a file gets corrupted.
I've done what you have done though. You might try making the file the larger pixel dimension to see how much it was degraded, I've been pretty impressed with how well PS rescales images, even from lo- to high res.
Sorry I don't know of any cure.