If you've only made one edit since clicking on the original snapshot, I think you might be ok. You should be able to just do Command-Z to undo, and your old history should show up in the history palette again, but grayed out. You should then be able to click on the last one in the list and have everything back. If you've made more than one edit, though, I think you're stuck.
Disclaimer...the machine I'm on right now has CS3, and I tried it with a sample file and that's how it worked for me. I don't know offhand if newer versions are different or not. Please don't come after me if it doesn't work 😱