This might have been an interuption in service from your host and their DNS servers. I had a problem once where my sub-domains vanished for 10 hours once
Do you get any e-mails sent to you of errors or any in log files? EX:
If my mysql server errors on anyone I get a e-mail from the site forums telling me there is something a miss.
If this does not clear up and starts to progress.
Take a look @ your host's support area for news of upgrades and also network status.
I also found this: