Good idea to see if the neighbours can check where the kittens are and how safe it is.Clearly something spooked her when it was time for her to have her kittens, so she had them elsewhere. She must be getting food from somewhere, but maybe not enough.
Can you ask your neighbor to get a ladder and you guys look on the roof for the kittens? They may be in a gutter, which is not safe if it rains. Or maybe they are in the bushes at this neighbor's house. You need to try to locate them now that you know she is around.
If you find them, you need to have a box ready to put them in and bring mama and babies back to your house.
Let us know how it goes. Good luck!