This problem occurs when one of your tables get corrupted which can happen because of any reason – can be the one you never expected like mishandling of packets while insertion to tables, sudden disconnection to database, erroneous spam comments or posting error from your desktop blog editor.
In order to fix it, all you need to do is to go to your MYSQL database (prompt or phpMyAdmin), run the REPAIR TABLE command –
repair table wp_comments repair table wp_users
That is it.
Thanks Anirudh for instant intimation.