General maintenance
Monitor the water level of your pond, as hard frosts can cause defects in the liner and in concrete structures. If the water level drops considerably, then it may have developed a leak. Be sure to keep it topped up until repairs can be carried out in the spring.
Rake out fallen leaves or shake off those that have gathered on protective netting.
Troubleshooting
If necessary, use pond heaters or place floats on the surface of the water to keep it from freezing over as this can be fatal for fish and other pond life. To make a hole in frozen ponds, hold a saucepan of hot water on the surface until melted through. Do not crack the ice, as this is harmful to fish.