So you just did File->Delete Version (or pressed ⌘⌫) and not only is the version gone but so is the master! What happened?
File->Delete Version (⌘⌫) is terribly misnamed it should be Delete Version and Possibly Master. While it does delete the version it also deletes the master if this was the only version of that master . What is even worst is that no additional warning is given.
So what to do? In this case the version and master are still in Aperture's trash if you haven't emptied it. If they are there you can click on the trash icon, select the image and select put back from the menu. If you have emptied the trash they will be in the system trash and you will need to import the image again.
To prevent this from happening in the future remember if there is only one version of a master in the library then deleting that version will also delete the master. If you want to get back to an unadjusted version of the master you can either remove all adjustments from the version or else duplicate the master (not the version) and then delete the adjusted version.