A. Nothing, is probably the best answer.
The market volatility we saw on Friday would be barely noticeable if you have longer term time horizon. Sure, some stocks or funds were down as much as 10% on Friday, for example, the Vanguard FTSE Europe ETF was down 11.3% on June 24, 2015, but that was only the eighth-worst monthly fall in the fund's history, making the daily fall on Friday not as frightening as it appeared.
If you did feel the pain, here is one thing you should consider - your investment maybe too aggressive and beyond your risk tolerance level. It's probably time for you to reassess your risk tolerance level and build up a portfolio that appropriate to it.
If you are a patient long term investor, the more severe such a drop of the market, the happier you should be - because a rare buying opportunity just presented itself in front of you.