Hi!
Did you change settings in the "Configuration" area of your blog, or in the "Personal Preferences" area of the blog?
If you changed something in the last one, then you set the option "Forbid creating entries" to "Yes". This makes your account unable to create any more entries, as the description of that option tells you.
The easiest thing for you would be to re-install serendipity. Delete the serendipity_config_local.inc.php file and drop all serendipity_* database tables, and then you can easily reinstall it (without the need to reupload all files).
The harder way to fix this is to search this forum for "no_create" and follow the SQL commands outlined in those threads.
Best regards,
Garvin