When you have to face the music, you’re confronted with the consequences of something bad you’ve done.

For example:

If your boss catches you lying about what time you got to work, you’ll have to face the music.

I told you not to try to sneak in, and now that you’ve been caught,

you’re just going to have to face the music.

If we do nothing to curb this pollution, I guarantee 

we will face the music in the future.

Now, you’re going to have to face the music for your 

poor judgment—expulsion from school.

Tradução:

To face the music significa “encarar as consequências”, “sofrer as consequências”, ou “encarar a realidade”.