Unfortunately, I don't know of any general way to revert your file back to an earlier state (once closed) unless you had some sort of automatic backup system in operation.
If, by some chance, there were only a few vector objects in your file and you only rasterized them, but didn't flatten the layer stack, it's a lot less work than trying to reconstruct an entire layer stack.
I probably don't need to tell you this, but if, as you were working on the project, you had saved multiple copies of the PSD file, each with the date and time in the file name (eg, Grandma_portrait_recolorization-2014_01_21-1715z.psd) you would not be facing such a problem now. If you are worried about the space on your HD this requires, you can delete the oldest versions or the versions reflecting small amounts of progress when you are all done. Another option is to email the intermediate copies to yourself (or use dropbox or something similar for big files). This has the added advantage of providing an off-site backup should a fire or something awful happen to your computer or your office.
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