If you are trying to log into Twitter.com or check your Twitter account on a mobile device, it's likely that you can't do either right now. At the moment, the Twitter website is down and both first and third-party Twitter apps can't send or receive messages.
Twitter users are used to seeing brief amounts of downtime for the social networking service, with error messages that show that the servers are over capacity. However, it's rare that a long period of downtime, such as the one that's happening today, occurs.
So far, Twitter has yet to comment about the downtime. It could use its Facebook page to issue a statement but so far it has yet to make such an announcement.
Update: Twitter has posted a statement on its Status page (which is apparently still working), which says, "Users may be experiencing issues accessing Twitter. Our engineers are currently working to resolve the issue."
Update 2: Twitter appears to be back up and running, at least at the moment. Twitter has yet to say why their servers were down.
Update 3: Twitter appears to be down yet again ... hmmm. Twitter's status page now says, "The issue is on-going and engineers are working to resolve it."
Update 4: Twitter.com appears to be back up, at least for now.