Just a few things then!
Assuming you can’t access any admin pages, I’d probably start with the plugins directory. FTP into your site. Go into the wp-content folder and rename the ‘plugins’ directory to ‘plugins2’.
Try and login. If you have success, you’ve found the problem, so you’ll need to rename your plugins2 directory back, then using a similar process of elimination, hunt down the offending plugin located inside that directory.
If still can’t login though, check your wp-admin folder. Presumably it is still all intact? Delete it entirely and re-upload it from a fresh copy of WordPress, just in case.
If still no joy, I’d probably fire off an email to the webhost, just to see if it’s some bizarre mod_security issue, blocking you from accessing the site.
Report back how you get on. I’m off to bed now, but I’m sure someone will chime in with other ideas if required. Best of luck!