Don't panic if you get something like this error:-
Fatal error: Can't open file: 'sessions.MYI' (errno: 145) query: SELECT u.*, s.* FROM
users u INNER JOIN sessions s ON u.uid = s.uid WHERE s.sid = 'eb41686f3dc5941ad09e162d1e
53088e' AND u.status < 3 LIMIT 0, 1 in /home/yoursitename/public_html/includes/database.
mysql.inc on line 66..

This is because mysql 'users' database table is corrupted and needs repair. This is not a Drupal problem. Read thread here.

MySQL Account Maintenance

Go to your cPanel. Open a web browser (Internet Explorer or Firefox) and enter directly into the url address box:
http://www.my_domain.com/cpanel.
Press Enter or Select 'Go'.

Enter username, password

Select mysql> 'Check' [and message like below appears..]

Checking Database
yoursite_drpl1 is now being checked.
yoursite_drpl1.access OK
yoursite_drpl1.accesslog OK
yoursite_drpl1.aggregator_category OK ...
..................................
yoursite_drpl1.sequences OK
yoursite_drpl1.sessions
warning : Table is marked as crashed
warning : 1 client is using or hasn't closed the table properly
error : Found 244 keys of 245
error : Corrupt
yoursite_drpl1.system

Now Clicked 'Repair' and you should see all tables as 'OK'.