angelfish behaviour - post egg laying

i wonder if you could help with this? I had a spawn about 2 weeks ago, all was going ok. a few days ago the pair showed signs of spawning again. then i awoke one morning to find all the babies gone - eaten by one or both parents . the morning after, fresh eggs. now the male is guarding the eggs and wont let the female near them and he's been agressive towards her. they have never been like this before and usually look after the eggs together. any ideas why the male has had a sudden change of behavior?

Female angels need to eat food to convert it into eggs before they can lay eggs. In the wild food is quite scarce, so whilst guarding a shoal of fry (which many other fish might regard as a good lunch), a female is unlikely to find sufficient food to become ripe and ready to lay.
The situation in an aquarium is very different, with the aquarist delivering copious amounts of food at regular intervals, and it's quite common for females to become heavy with eggs whilst still guarding the previous batch. In some cases they lay and attempt to guard both, usually when this happens the fry start to eat the eggs, now the parents are conflicted, they want to defend their fry, but also their eggs from potential predators, yet their fry are themselves predating the eggs.
From what you describe I suspect the female's desire to defend the eggs was stronger than her desire to defend the fry, now the male blames her, and won't let her near the eggs in case she eats those too!

Chances are that things will settle down in a day or so, but keep an eye on them in case the male is too aggressive with the female. In future, watch out for the signs that the parents are preparing to spawn again, and remove them, or their fry before they do!