Cloudflare

Cloudflare is a really handy service for anyone who is experiencing DDOS attacks.
Cloudflare offer DDOS protection and security solutions.

They offer a free option which will provide you with an I'm Under Attack Mode option. If your website has gone down because of a DDOS attack you can activate this mode and it will block the attack and your website will be available again.
Cloudflare also offer priced monthly plans with extra features, but I think every website should at least be on the Cloudflare network under the free plan.

Another benefit is that you can improve the performance of your website by serving up content through the Cloudflare content delivery network.
I highly recommend looking into this service if your a website owner or administrator.