Why doesn't the sound on my MacBook work?

When I try to change the sound, it shows the sound bar full and white, but the big icon above it shadowed out, and there’s a circle with a line through it below all this. The sound level bar won’t change, and no sound will come from the speakers. However when I plug headphones in, the sound works fine.

I’ve checked system settings, and I believe that the internal speakers are selected for output. idk.

Thanks