Livebearing females can hold their fry until water conditions are right. Large, frequent water changes are important. Floating stem plants will help the females feel more secure. A teaspoon of standard aquarium salt added to every 5 gallons of the new, treated tap water is also helpful. You may just need to prepare the tank a bit. You'll be rewarded with new fry.
"Fear not, my young apprentice. Just change the tank water."