My website isn't working: "Craft does not support backtracking to this version. Please upload Craft 2750 or later." I have been unable to access both my website and my login page. What's going on?
Sounds like your database was updated but your filesystem was not - that could happen perhaps if the development version and the live version share the same database, and only the development version ran the update.

To fix it try renaming the craft/app folder to craft/app-old, then copy the craft/app folder from the latest Craft download into your site.

Backup your stuff first.