Sadly, this does happen a lot [virtual or not]. When this happens, I wait for her angry-ness to melt away. But if I seriously want to talk to her/him, I would apoligize first. Even if it wasn't my fault, some people think you did something when you really didn't.
Once I was talking to 3 people at once and they all said something to me at the same time. I answered them the best I could but forgot to say who I was answering. So one friend got mad at me for saying something mean to her when I wasn't talking to her. [She said: Does this look nice? and I answered someone else at the time saying: Not really. [the other person asked if my sister was watching the screen].]
Accidental talking like that does happen a lot. And I ended up sending an entire message saying "I'm sorry" over and over and over again. Not kidding. Perhaps ask your friend what happened to make him/her mad at you? They may or may not say but if they don't cool off from their anger in less than a week, something may be up. Maybe ask if anything has gone askew in their life? Events outside of friendships do play a hand in how a person acts and thinks.