Did she leave you a message? If not, I don't see what the issue is. For all you know she might have misdialed.

Option 1- don't reply.
Option 2- send a text "I just noticed I missed a call from earlier, I'm available now if needed."

I like option 1.

Me:38 XW:38
T:11 years M:8 years
Kids: S14, D11, D7
BD/Move out day: 6/17/14, D final Dec 15