Try REPAIRing the wp_comments table using phpMyAdmin as that table might be corrupt.
Well I have never used phpMyAdmin before, so what would I repair?
The table giving you the error.
It might be a good idea to contact your host and see if
they’ll help you.
I had this same problem after needing to reboot MySQL.
If you have cPanel, all you really need to do is go into cPanel, find the link to MySQL and once you’re in there you should be able to locate your database with three options beside it: Delete, Check and Repair.
Click Repair and if all is good, you’ll be fixed.