WordPress.org

Forbidden
You don't have permission to access /blog/wp-admin/post.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
I haven't changed any permissions it was working just fine and all of a sudden that error came up..

Try changing post.php's permissions to 755 (CHMOD 755). If you don't know how to, this might help: http://www.tamba2.org.uk/wordpress/ftp/
If that doesn't work, then it's probably something that your hosting provider did. Send a support email to them.

Thanks I'll try that...

That didn't work..I contacted my host(i send a trouble ticket and posted an inqury on there forums) In the mean time has any else got any more ideas :) ??

I'm having this problem too. Can't edit, can't post and can't leave comments. CHMODing didn't help.
Were you able to fix the problem??

Turns out it was the PHP upgrade/Zend Optimizer thing. My host fixed the problem for me.

My friend's blog is having the same problem. I chmodded her post.php. My host upgraded the zend thing. still that same problem.

it seems the 403 thing is going around.
see here.