You have a different problem from the one described in this thread. This thread is relevant to the situation where you get the message:
No Update Required
Your WordPress database is already up-to-date!
In your case, since you see a blank screen you would need to look at your error logs, or php error logs, and open a new discussion threat specific to what you find.
Excellent suggestion. The old theme and all plugins might work, just a matter of upgrade order issue:
Switching to twentyeleven, then upgrading the plugins, then back to the original theme, and voilà, admin section is working again.
