The baby unit and the parent unit are too far away from each other, and the connection got lost
Move the parent unit closer to the baby unit.
The baby unit may be switched off
Switch the baby unit on.
The baby unit or parent unit is too close to another DECT appliance (e.g. a cordless phone)
Switch off this appliance or move it away from the baby monitor.
The rechargeable batteries of the parent unit are running low
Connect the parent unit to the mains and recharge the batteries.