I've had this happen (though, luckily, I don't keep albums with 7000 pictures!) I'm afraid you will need to resort.
Manual sorting of pictures in an album works but, since there is no way to reproduce the order, if something disrupts the order, you're stuck.
I have decided to put titles on my pictures that include the order, and then sort by title. I use Photos Workbench ($29) to batch change the titles once I've put them in order. Photos Workbench works like this:

I put the pictures in an album in the order I want, and then I give them titles indexed to that order. There are many choices that you can arrange in the title as you want. Then, even if the album is lost or they just move around, you can re-create the album's order.
I do use albums with thousands of pictures when first organizing a set, but I keep them in smaller albums inside of folders to make it easier to manipulate.