First thought: why did you only have one save game? That's never a good idea in RPGs: even if you're not worried about file corruption losing it (and this is a danger in even the most bugless computer game) in RPGs it's quite likely you'll do something you regret at some point and want to backtrack to before your latest save. Take this as a learning experience, and always have multiple saves in an RPG.
Now, to deal with this particular case, we'll probably need a lot more information. How did you "lose" it? Did you accidentally delete it (in which case the file may be in your computer's recycle bin)? Did the game crash and make the save unopenable/unrecognised?
Do you have an autosave/quicksave slot filled? The autosave should be from the last time you left an area, so try loading that if it's present. Then make a full save from it immediately, in case you mess up the autosave. I somehow doubt you know about the quicksave button from what you've said: for future reference, you can press Q to save the game in a quicksave slot, which saves time.