The error message is very clear. It means the database configured in your connection string in Web.config or user.config is not correct or the database server is not accessible from the web server.
mojoPortal doesn't change your connection string and it can't control whether your database server is accessible from the web server.
If it was working last night then either someone changed the conneciton string or something happended at your host and the database is no longer reachable. The first scenario is more likely, like if you uplaoded a copy of Web.config or user.config with an incorrect connection string.
See also the documentaiton about Using MS SQL and Moving an Installation of mojoPortal to a Different Server.
Hope that helps,
Joe