Besides cleaning it, sometimes just breaking the electricity for a moment, and also SMACKING the pump will get working again.
My fish are NOT happy very long at ALL without aeration. I've read here the suggestion of keeping a back-up pump, maybe not big enough to run the filter, but big enough to get water splashing and moving.
In the meantime, I would go out there every now and then and scoop up buckets of water and splash them back into the pond, to try to get oxygen down into the water.
Do it beFORE the fish start gasping for air.