Hi there and
to the forums,
First of all, take a step back for a second. Budgies do not get "pregnant", nor can babies be "stillborn". Budgies are birds and, as all birds, lay eggs, which are incubated for several weeks before a baby is fully formed.
As mentioned, you don't seem to have any of the experience necessary to breed your birds. Breeding budgies is not as simple as just throwing in a nestbox. You are endangering your budgies by allowing them to breed, so you need to remove the nestbox right away and rearrange the cage.
It seems that your female has some kind of medical issue, potentially egg binding or prolapsed cloaca, both of which can become fatal in a very short time. You need to take her to an avian vet to ensure that nothing happened to her during the fall. Additionally, you need to take precautions to ensure that no breeding occurs.
If she does lay an egg, you need to discard it immediately. This is no different than throwing away an egg from the grocery store. No life is formed in the egg until several consecutive days of incubation. Until then, it's just a regular egg with nothing in it.
Please let us know how your budgies are doing soon!
Additionally, be sure to begin reading through all the budgie articles and "stickies" (threads "stuck" to the top of each subforum for easy reference) to ensure you're up to date on everything. If you have questions after reading though things, be sure to ask.