I had a client call yesterday saying their website was down. Upon investigation, it was discovered their domain had expired. With a quick phone call to their registrar, they were able to renew their domain. As of writing this, their site had been down for a few hours. Sadly, domains don't spring back to life once the bill is paid - there's always a bit of downtime involved.
Why did this happen? The main reason was that nobody received notification the domain was coming up for renewal. Most registrars send email reminders about renewal dates. In this case, the administrative email address for the domain was incorrect so no reminders were received.
If you're managing your own domain name, ensure your administrative address is correct. If it's not, fix it right away. You can check what address is on file by logging in to your account with your registrar.
At The Web For Business.com, we manage the majority of our clients' domains so we get the domain renewal notifications and can renew accordingly. It's a small but vital service. Just something to keep in mind when you're choosing a company to deal with for your website development and hosting.