Please give the 3.5 Master List a read.
If that is it then try these steps (which I've copied from an earlier reply).
- Make a backup copy of your
wp-config.php file and put that somewhere safe.
- Copy that
wp-config-SAVE.php (I really like multiple safety nets).
- Open the
wp-config.php file using the most basic text editor you have. Windows notepad is good for this and add this line above the "stop editing" line so it looks like this:
/* That's all, stop editing! Happy blogging. */
Save that, clear your browser cache and cookies (don't skip this step, it will not work if you don't clear that out on your browser), re-login and try again.