This is apparently due to problems with their server and datacenter. The good news is they appear to have it ready to bring back up (at least in some form) by tomorrow morning according to this post on their forums:
“good news from Crazzykid. Power won't be up until Wednesday or Thursday, but he's going to the co-loc early this evening (a few hours from now) to get the server. We'll get it online tonight. Tomorrow he'll be moving the server image onto his SAN and a production VMWare server. That will allow us to increase the ram to about 16gb at the same time. Once it's all moved, the current server will be used as an additional mirror box so we'll increase our redundancy a bit in the near future.”Since thetvdb is the main TV scraper for SageMC, XBMC and countless other HTPC programs, you likely can’t update the metadata for those TV shows for now. To keep up with the progress on the site watch for status updates at their forum. And if you really miss them, you might consider a future donation to the cause to help them continue to “serve” the masses.