Hard plastic pond liners are normally made of PVC. They are preformed and sit in the ground inside a specially dug hole. Since they are not very flexible, any movements in the ground such as settling or frost heaving, can cause damage. They can also become brittle due to weather elements. If the crack is small it is possible to repair it. However, once the plastic is cracked it will continue to crack and eventually need to be replaced.
Drain the pond until the water level is lower than the crack. If the crack is low in the pond, you may need to temporarily remove the fish and plants.
Clean the area surrounding the crack with a clean, wet cloth. If you need to scrub with a bit of abrasion, use some dry salt. This will not be harmful to either your fish or your plants.
Use a blow-dryer to completely dry the area. Do not hold the dryer on the same place for too long and make it too warm. This could cause a larger hole in the liner than you started with.
Apply PVC sheeting cement to a repair patch and over the crack. Wait for three to eight minutes. The length of time you should wait depends on the air temperature. The warmer it is, the less waiting time. The glue is ready when it is no longer tacky.
Press the patch over the crack. Smooth it out so there are no wrinkles. Press it in place for at least 30 seconds.