If Sc2gears is running, by default it auto-saves replays that are played at the time.
If you were running Sc2gears, it made a backup of your new replays. By default they are saved to "C:/SC2Replay Archive" (but this is changeable).
Check the Folders tab in the Miscellaneous settings dialog to see where it saves replays.
If you did not ran Sc2gears while playing, then the only thing I can recommend is some utility program that can recover deleted files (if you did not overwrite their placed on the hard drive). Google it.