It sounds like your AUTO_INCREMENT property was cleared from that table.
READ THIS WHOLE POST BEFORE DOING ANYTHING.
If you are comfortable with running SQL statements, you can fix this yourself by following these instructiosn. Otherwise you should ask your site administrator to do the following:
1. Run this select statement:
SELECT MAX(ID) FROM wp_posts
2. Add 1 to the number you see on the screen and write down the new number.
3. Run this next SQL statement using that new number instead of ###:
ALTER TABLE wp_posts SET AUTO_INCREMENT=###
You're done.
Of couse, if you didn't use wp as your table prefix (look at wp-config.php in your blog root to double check), then you should replace wp_ with the correct table prefix.
Also, case matters. WP_posts is NOT the same as wp_posts or WP_POSTS. Type it with the correct case or you'll get an error.
Test this fix by creating a new post. It should have the number you wrote down as an ID.
Hopefully that will help. If you have any questions (other than "How did this happen?" - I can't answer that one) let me know. I'll try to check back on this one...
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Brian Layman
http://www.TheCodeCave.com