Whether you need to reheat an entire bread pudding you've kept in the freezer or a single portion of leftover bread pudding from the fridge, you have two options -- the oven or the microwave. What's most important in either case is heating in a way that doesn't dry out the creamy dessert. Although you can serve leftover bread pudding cold or at room temperature, it tastes better and feels richer and more creamy in your mouth when you serve the dessert warm.
From the Refrigerator
Leftover bread pudding or bread pudding that you've made in advance both stay safe in the refrigerator for 3 to 4 days, but they stay more moist if left only 1 or 2 days. To reheat bread pudding in a preheated oven set to 350 degrees Fahrenheit, cover it with foil and heat it for from 5 to 15 minutes depending on the amount of pudding. Microwave bread pudding on low power for 2 to 10 minutes, checking it often. For either method, heat the pudding until its temperature reaches 165 F, checked with an instant-read thermometer.
From the Freezer
Frozen bread pudding that you've baked ahead of time will keep for 2 to 3 months in the freezer -- cool the dish in the refrigerator before setting it in the freezer. Then, when ready to use it, set the frozen dessert overnight in the refrigerator until it is completely thawed; check that it's completely thawed by inserting a skewer or knife in the center of the dish. Reheat thawed bread pudding either in the oven or microwave in the same way you would leftover bread pudding.
Storage and Reheating Tips
Because bread pudding can sometimes come out dry to begin with, if your eggs were on the small side or the bread soaked up too much of the milk mixture, you need to store the dish in a way that minimizes it drying out. Cover it tightly with plastic warp in the refrigerator and with a few layers of foil for the freezer. If you like a crisp top on your bread pudding, place it in an oven-safe dish under the broiler for 1 minute after reheating in either the oven or the microwave.
You'll know if your bread pudding has dried out if all the custard portion of the dish has disappeared -- a result of the bread soaking up all the moisture. In this case, add a sauce, ice cream, whipped cream or a splash of heavy cream just before serving the warmed dish. Sauces that work well in bread pudding include any store-bought caramel sauce or creme anglaise, a custard sauce made with 2 eggs for every cup of cream or milk and every 1/4 cup of sugar.