How old to start with? and it does sound like something may have scared them. is you pond dug into the ground? if so you may have a snapping turtle and they can do some serious damage even kill ducks.
I think you solved the mystery, they may have been in the pond when the big monster flew at them[pond liner] I don't know how you feel about walking into the pond this time of year but that may get them motivate enough to follow and see that the boogie man has been eliminated. Ducks scare pretty easy being preyed upon by some many even pond liners. If you have dried meal worms they will float and might also get them motivated in trying it again.
I am thinking the pond liner scared them and they are afraid to go in now, Today I changed my reg hose out for one of the kind you see advertised that collapses when you turn off the water and drain? it's black and when I turned on the water and it started moving to fill up There was a panic among the flock I am thinking they thought it was a blk snake since we have a lot of them and they come through the yard a lot in the summer. It was funny to me but not to them. Oh and forgot to say
Let us know what works and when you get them back in, there have been others that have had similar trouble so it is always good when some one actually gets them back in the pond after a scare. and how they did it.
