What does it mean when a girl texts you a sad face and hasn't talked to you since?

I've been seeing this girl for about a month now and last week I didn't reply to her text as I was busy. Two days ago she sent me ':(' and nothing else. I replied back straight away apologizing asking what she was up to but she didn't reply back and hasn't contacted me since...

What is on her mind? Have I messed up by not replying to her text for a few days?
What does it mean when a girl texts you a sad face and hasn't talked to you since?
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