Why did my zebra finches abandon their nest?

My two zebra finches have spent the last three days building a nest inside a nesting basket I bought at Petco, with bits of cotton and nesting material that I also bought there. They slept in there together every night and now today, they're huddling together on the other side of the cage! It's like they're totally ignoring the nest now. I don't know whether they still want to breed or what?