Rice dries out fast in the fridge, which is exactly why it clumps into a hard mass when you try to reheat it straight from cold. The fix is almost always the same: add water back, then trap the steam so it can actually rehydrate the grains.
Break up any clumps with a fork, sprinkle a tablespoon or two of water per cup of rice over the top, and cover the dish tightly (a lid, plate, or damp paper towel all work). Microwave on 70% power for 1-2 minutes per cup, or heat in a covered pan on the stovetop over low heat for about 5 minutes, stirring once halfway through. The trapped steam rehydrates the rice instead of just heating dry, dense grains.
Add the rice to a saucepan with a splash of water or broth, cover, and heat over low for 5-7 minutes, stirring occasionally to keep it from sticking to the bottom. This method works especially well if you're reheating enough rice for the whole family at once.
Rice carries a bacterium called Bacillus cereus that can survive cooking and multiply if rice sits at room temperature too long. Cool rice within an hour of cooking, refrigerate it promptly, and reheat it until it's steaming hot all the way through. It's generally fine to reheat rice more than once as long as it's been stored and reheated properly each time, but it's safest to only reheat the portion you plan to eat rather than repeatedly warming the same batch.
Skipping the water and the cover is the single biggest reason microwaved rice turns hard and clumpy. The microwave dries out whatever moisture is left in the rice instead of adding any back, so the grains just get hotter and harder rather than softer.
Cooked rice keeps for 3-4 days in the fridge and up to 6 months in the freezer if portioned into flat, sealed bags before freezing.
A simple side like cinnamon basmati rice reheats well with just this method, since the extra splash of water brings back the fluffiness that steaming rice depends on.
For more on reheating other meal-prep staples, see the full reheating guide.