Yes.. It's our hosting company, we believe.
After talking to xfir he checked the error log and found that it wasn't able to start sessions (in layman's terms, one of the things you can't do very well is log in) because the temporary folder on the server isn't writable. The mySQL database also uses the temp folder, so that's why you're getting the error.
He's opened a trouble ticket with the company, which hasn't done anything for us yet as far as I know.
He did mention they were upgrading some servers, so that may be the cause.... but this has been the 3rd day now....
I will talk to him and we'll try to get this forum back online.