I've been Mia since this summer, I believe. I recently logged in to a bunch of notifications of gift boxes, and can't seem to find anything. :( Is this because I've been gone for so long?

The Gifts page for gifts you've been given is now organized by month, so if you haven't been given anything so far this month then nothing will show up at first. You can select different months/years from the drop down bars at the top to find the gifts you've been given since you were gone.

No, those gifts should still be there. The gift page is now paginated though, you will have to go back month by month to access the older ones.

Near the top of the gifts page should be some drop down boxes with dates Gifts Inbox: October 2017 Jump to month:

[edit] Guess I should refresh before posting!