Doing a manual downgrade back to 2.7.1 should fix the problem. If you can, back up your contents even if you have to do it via FTP. I have been looking all day but there is no easy way to downgrade. Why they don't have a revert back to previous version script is beyond me. I would think it would be easy. But I have yet to find one.
Anyway, got it back up to 2.8 and my admin is working but none of the plugins work at all and they hose up the site big time when any of them are activated. So going to have to do a manual downgrade to get back to a working version. Yippeee....
Rule number 1# Always back everything up. #2 Never upgrade in the first month or two. #3. New and improved can be a big fat fib... :)
Good luck and I hope you get it fixed. Took about 4 hours to get it even half way back to normal.
Just go up to search and look for downgrade. It is a bit of a pain in the butt process. Getting ready to do it here again.
If you come up with a better solution, please let me know.