The error states that the POSTS table has crashed. Very bad news.
Unless you have a backup of the database. You only option is to attempt a repair manually.
I can only assume that you can't access your ubb control panel and you have phpmyadmin in your hosting control panel.
If you do then there is a tool to check the table as well as repair it.
From the host control panel select phpmyadmin.
Open your ubb database.
select the ubbt_POSTS table on the left.
Then select operations from the tab on the top.
On the bottom left is Table maintenance
You have several options of which repair is one.
Also there is a icon to the right of each option that explains the process to you which I suggest you do.
Run the repair when ready.
If you have ubb control panel access.
on the left near the bottom should be Database Tools
Select it and then select the SQL command tab.
Enter
REPAIR TABLE `ubbt_POSTS`
in the query box.
Then click on execute.
You will get a brief message and hopefully it should have a ok.
Good luck
