Check if you ran out of disk space - this sometimes affects the session manager.
Reset your file permissions/ownerships to correct values - broken layouts are usually caused by bits of cached CSS not loading.
Tip: if you go to Admin - Upgrade and run the first step of the wizard you’ll get a report on the state of your permissions. Then you can cancel without running any upgrade.
Are you on a Linux or Windows machine?
If you are on a Linux machine search the forum: there are plenty of post regarding permissions.
I have also posted a small script that sets them correctly for you that you can run from your browser.
Once you have set permissions correctly you should also log in, go to Admin and run Quick Repair and Rebuild. Additionally you may have to rebuild the jascript files as well as .htaccess
I recommend a full back up (db + folders and files) before doing anything