For your own edification, a large number of people logged on would have nothing to do with a DDoS. A higher than normal number of users logged in does have the potential to slow down a website. Perhaps user accounts were not being properly logged off which would increase the load on the system. This could simply be an issue with a script that went haywire which began consuming resources on the web/DB server.
A DDoS, also known as a distributed denial of server is would not increase the number of users being logged in but may well slow down the site or entirely take it offline. McDude is correct about needing equipment to stop a DDoS. These are sometimes called scrubbers. They use a variety of methods to stop the DDoS including changing the URL by adding random characters at the end. Black Lotus makes equipment which is designed to handle DDoS attacks.
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