What can I do if I have lost my .iba file?

While you can extract some of the assets from your .ibooks file (it's just a zip file), you'd still have to make another book and put them in manually. If you did manage to get that far, I'm not sure the delivery process would accept it as an update, tho.

You may want to contact Apple support before you put in all that time.

And as always, have a tested and reliable backup in place when working.

I don't see what this has to do with iBooks Author in that example, sorry.

You might want to try the Delivery forums instead.

Good luck in any case.

Only iBooks Author creates an .iba file. You didn't use it, so you could never have had one. You need to find whatever it is that Sigil uses to save its projects before it converts things to epub. Ask in the Sigil forums