iPhoto "could not save changes"... then ENTIRE library deleted! Help!

Hello All!

IN BREIF:

I was consolidating my photos in iPhoto, which took over 3 hours to go from around 8000 to just below 5000. Right after my FINAL change of this feat (just adding some childhood photos to an existing event), I got a notification close to the following "Some of the changes could not be saved. Please check to see if you have sufficient hard drive space"... or something of this sort. Hard drive space would not be an issue on this computer - when I open Finder, it says "47.02GB available". I go back to my events, which no longer have main photo icons, but when I open them the photos are there... next this I know, "No Photos" at the bottom of iPhoto and they no where to be found.

A few additional details:

- My Macbook is nearly 7 year old (bought Fall 2008, the old white chunky Macbook). I was consolidating to transfer them to my new MacBook air, purchased today.

- No Time Machine back-up, only an external hard rive back up (which is currently at my apartment in another city)

- I just got back from 5 months abroad, during which my MacBook remained in Canada unused. I was happy to see that it still turned on while connected to a power source, but overall I have little confidence in it to perform in a way I can rely on given its age.

Further question:

- Would my back-up on my external hard drive be able to be opened on my new MacBook Air, even though it is a back up from a laptop that had a much older operating system? How can I make this work?

I have a lot of sentimental memories in my iPhoto, including irreplaceable photos of my dad who recently passed away. Would be eternally grateful to get them back.

THANK YOU IN ADVANCE!