There are several possibilities to enter your home without a smartphone:
You can still use your existing, mechanical key.
You can ask another authorized person (friends, family) to open the door for you. If you have a Nuki Bridge, this even works from afar.
You can use the Nuki Fob. The Nuki Fob is a small Bluetooth device with which you can lock your door without a smartphone.
At a power level of 20% Nuki will notify you via the Nuki app and directly on the Smart Lock. So there is enough time to replace your batteries of your Smart Lock. More information regarding the batteries you can find here.