My domain expired, how do I renew it?

DynDNS helpfully publish a guide to the options since they vary according the registrar and how long it's been. For example with a .com domain you have about 2 months from expiration before the domain is released, with .cx it's released immediately. If your domain hasn't yet been deleted you can simply renew it, though extra costs may be involved. If it has been deleted and nobody has registered it you can simply re-register it.

There are some things you can do to protect yourself in future.

1. Enable automatic renewal for your domain registration (see this answer for how to do that)
2. Put a recurring entry in your diary a month before the domain expires to remind you to renew it (just in case your registered payment option has expired)
3. Check the email address in your account and ensure that your anti-spam filters (on the mail servers and mail client) are configured to allow the email through