Why would a digital audio workstation expect audio???

Unexpected request for audio

This just happened while working on a song. Hasn’t happened ever. Sometimes I just don’t understand software. How can it work perfectly for years (literally) and then all of a sudden an error pops up that I’ve never seen. Even though the song I was working on played fine just a minute ago and I didn’t even make changes on the audio.

Can someone explain this to me?