WPyogi on "Warning: Cannot modify header information erorr leading into parse/syntax errors"

Are you stuck entirely -- out of the old theme too now? If so, you could try accessing your site via FTP -- and rename the bad theme folder -- then it should automatically revert you to the default WP theme and you should be able to then activate your old theme.

http://codex.wordpress.org/Login_Trouble#Deactivate_Theme