There are several backup systems in blender:
Temporary files (saved every 2 minutes by default)
These will be saved in your temporary directory (specified in User preferences > File) with a random number as the name.
You can quickly open an auto saved file by pressing File > Recover auto save, then selecting the auto saved file (the modification times can be very useful for determining which file to pick):
You can adjust the interval at which these are saved in CtrlAltU User preferences > File > Auto save > Timer:
Temporary quit files (saved on quit)
These are also saved in your temporary directory, with the name
You can quickly open it one by pressing File > Recover last session:
Persistent save files for reverting accidental/unwanted saves (saved when saving)
These are saved as
.blend2, etc. files in the same directory as the current .blend.
.blend1 will be the file as it was the save before your last save, .blend2 will be the save before the save before your last save, etc.
To open these files, navigate to the directory where your file was saved and enable backup files in the display filter:
You can configure the number of these to save in User preferences > File:
Without knowing more about your particular situation (how long it's been, has tmp been cleared, how often did you save, etc.), it's hard to say which will work best. There's no risk to trying them all, be beware that
\Temp is normally cleared out at some point, so get any useful backups out while you can.