I am having a very strange problem on a 3.9 , suddenly i can't access any

table

on the databases. I have around 100 databases on this server and can't access

not even one. This is a production server and i am in an urge to solve it, if

anyone

can help i would appreciate it:
# mysql mysql -p

Enter password:

Welcome to the MySQL monitor.  Commands end with ; or \g.

Your MySQL connection id is 303342 to server version: 5.0.18
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql> show tables;

ERROR 1018 (HY000): Can't read dir of './mysql/' (errno: 9)

mysql>
I have backups of all databases, including mysql database which i think is the

one

broken , how can i restore it ?
Thanks for your help