Hi today went out to check on fish and found pond down about 12 inches of
water.

How does this happen and how do I figure out what's going on?

Water level was constant for last couple of days. Checking the drain time
now very carefully. What should I be looking for?

If I have to replace pond liner, i have a rubber liner in a free form pond.
What is better a free form or a tub that I've seem in the shops.

Also how long does a plastic liner last??