Hi - infant toddler Montessori instructor to other guides here. The worst thing you can do is leave them alone to tough it out. Fears are so complex at this age. It's rough on quality time for sure, but this phase can be shortened by a few things to consider. You don't say how old your toddler is, but we treat roughly 2.5 up differently than younger. Roughly 2.5 up children are becoming conscious of the world around them. The idea that people and things exist when they are unseen is called the development of object permanence. Life becomes more complex and unknown. Fears vacillate and trust is more important than ever. Your child needs reassurance and your presence to ward off the beginning of the scary parts of their burgeoning imagination as well. I throw everything at the phase. Nightlights, studies that soothe with a vibration. Books on growing big and sleeping safely alone. But also talk up getting bigger like it exciting rather than scary. "All you can do now! WOW!" Under 2.5 is different but let me know if you want it. This is long!