I upgraded to 2.6 from 2.3+ using Fantastico. Been using WP for years yet I was not aware that there might be problems if I didn't do incremental updates :-(
After upgrade to 2.6 all my categories were missing. So I figured another upgrade to 2.6.1 would fix it. Dummy me.
Then I found the fix here.
I hesitated for a day b/c I detest going into PHPMyAdmin -- it's a real danger zone for those [like me] who don't really know what they are doing.
But, I followed the directions exactly and it worked - Great!
I happily begin tweaking away at my blog. Then get an email noting a comment needs approval.
All comments are missing from my blog!
So, back to WP Forums for the answer and it was covered here - albeit for WP 2.5.
All the comments on that WP forum post praised the directions to get the wp-comment section working smoothly. So I followed the instructions and went back into PHPMyAdmin.
Here are the instructions from eexlebots:
I had this same exact problem and jmfolcik's solutions worked for me; from phpmyadmin I selected the database, then selected the wp_comments table and viewed it in "Structure" mode.
Once there I selected the "comments_date_gmt" field and checked the "index" option. Once I did that and it reported a success I checked my WordPress site and comments were back with no errors!
What happened is I now only have the latest comments  -the ones that were awaiting approval. All comments from years past are gone.
I'm hoping someone here can help me get my full database of comments restored.
THANK YOU !
PS -- If it involves using a backup can I just import the Comments and not the entire blog? I would hate to overwrite my Categories again.
PPS -- And if you can post step-by-step directions I will be grateful!