If your website is experiencing a DDoS attack, then Cloudflare could help mitigate it.
The Cloudflare service has an option designed to be used when under a DDoS attack and you can enable it anytime you want from your Cloudflare account.
When this option is enabled, every visitor to your website will see a processing page for several seconds. During that time Cloudflare will verify and confirm if the hit is indeed coming from a legitimate visitor and not from a bot.
Also see: What is a DDoS attack?