I woke up this morning to a blank document tree
Sounds like opener lyrics of a geek blues song
Did you try to access the manager with more than one browser?
Did the hosting company change any apache/php/mysql settings lately? *
Were you moving the site (local to remote, or switch directories)?
Are the documents still in the database? i.e. frontend is working fine?
* There is a very restrictive setting that some hosting providers use (I’m sorry, but I forgot the name for it. I’m not even sure if it’s an Apache or PHP config): Turning that on will block Javascript in some instances. Since lots of Ajax / JS is used in the backend, maybe those settings are blocking the manager. This could explain the 2nd + 3rd error message.
In any case: I’d close the browser, clean the browser cache completely, and try again. Or try it with another browser. If this doesn’t help, ask your hosting provider if they upgraded or changed something recently. Some companies don’t deem it necessary to inform their customers about such changes...