Peter -- adding (I hope) the excellent responses already given:
One of the downsides of using Aperture is that "Photo" is no longer easily defined. When you ask, "So to restore one lost photo from a month ago ... ", you must define, in terms relevant to Aperture, what you mean by "Photo".
In Aperture you import image-format files. On import, Aperture makes a note of where your file is located (and by default stores your file where it wants). Thereafter your imported file is known by Aperture as an Original (prior to v. 3.3: a "Master"). Aperture also creates a text file to hold instructions on what metadata changes you make and what adjustments you make. This text file is called a Version. Aperture creates an image you can see, and displays this image to you as a thumbnail in the Browser, as a Preview in the Viewer (when Quick Preview is turned on) or as a fully-rendered image in the Viewer. Aperture refers to this (somewhat fitfully) as an "Image".
Here's what you need to know:
Original * the instructions in the Version = the Image you see.
When discussing Aperture it is helpful to differentiate between
- the image-format file you imported: the Original
- the text file of instructions that Aperture keeps in order to modify your Original to show you what you've done to it: the Version
- the picture showing you all you adjustments, with all your metadata applied: the Image
There is a one-to-one correspondence between Image and Version: one and only one Version is used to create each Image. Throughout Aperture, "Version" and "Image" are used synonymously to mean "the thing you see". Only "Version" is used to specify the text file of instructions.
(There are excellent reasons for this set-up -- primarily among them Aperture's non-destructive workflow and excellent performance doing the tasks we ask of it -- but they are beyond the scope of this thread.)
Some of the responses you've gotten here are about restoring your Original. Some are about restoring your Image. Those are, in Aperture, different tasks, dealing with different (or in one case, more) files.