If you have ftp navigate to your directory,
Copy your active theme to your desktop and delete the remote folder, this will reset to the default theme.
It might be a plugin so copying any plugins you think may be a problem to the desktop and deleting the folders will not affect data.
Then see if you can logon the the admin area, if you can then it is a problem with the theme or a plugin.
Re-upload the theme and check, do the same with the plugins, until you find the one causing the problem.
A quick search and I found this relevent 'version 3 upgrade' topic, it looks like you just rename the '__plugins' folder, create a new 'plugins' folder upgrade then just transfer and activate the plugins one-by-one!