There are reports of widespread outages for sevferal major online companies. Reports are indicating that this is a DDoS attack targeted at a major DNS Provider.
DNS provider Dyn.com is currently under heavy DDoS attack which has affected many well known brands across the Internet including Amazon, Netflix, Reddit, Paypal and many others.
This may affect your site if you use them for DNS or if you use any service, like Paypal, that uses DynDNS.
The larger providers seem to have worked quite quickly to mitigate this attack, but it is ongoing so it’s important that you’re aware of it and carefully monitor your website during this time.