For some children, falling asleep without an adult with them seems impossible. Perhaps your child is used to you sleeping in bed with them, or on a chair in the corner of their room. Although your presence may help them settle to sleep initially, if these same conditions are not in place when they wake up in the middle of the night, they may struggle to settle themselves back to sleep.
In order to help your child learn to fall back to sleep by themselves, you could try gradual withdrawal. If, for example, you usually sleep in the same bed as your child, try sleeping on the floor next to them. After three nights, move slightly further away. After three more nights, you could sleep on a chair next to the bed. Then try moving to the corner, then outside the room.
If your child cries and tries to get you to come into bed with them be firm. Avoid conversation and eye contact if possible. This may be very difficult at first but eventually your child will learn to fall asleep without you there.