I'm upset with my best friend
Why are you upset with your best friend?
Because she has been ignoring me.
It is important to try and forgive your friends when they may have made you upset. If they are intentionally ignoring you however it makes sense that you would be upset. Try talking to your friend and find out what the reason behind them ignoring you was.
It's not a big deal since I'm ignoring her too
If you and your friend continue giving each other the silent treatment, nothing will be resolved. Sometimes it's best to be the bigger person and reach out to understand what went wrong.