Anytime you hit 'done' or 'open' in ACR, the current settings are written to the raw file's XMP sidecar and will become the new settings for that file. If you want to save the current settings before experimenting with new ones, you can do so as there is a "Save Settings" command on the menu drop-down at the top right corner of the settings panel. But you have to remember to do that before hand, so it doesn't really help you in this case. If you still have the PSD/TIFF that was opened from the old settings, you can view those settings in the PSD/TIFF metadata to get the ACR settings, and then manually apply those to the raw file.