If you've upgraded from a MySQL version prior to 5.1, to a MySQL version of 5.1 or later, you might find some of your databases have their names altered to "#mysql50#dbname", where it used to be just "dbname".
This is because in MySQL 5.1 the data directory structure changed.
You can either alter your queries, so you retrieve from the database "#mysql50#dbname", or you can be sane and do the following query to upgrade the data directory structure. Note; this assumes you have shell access to the server with enough privileges! If you don't, please contact your server administrator.
mysql > ALTER DATABASE `#mysql50#dbname` UPGRADE DATA DIRECTORY NAME;
Note the backticks (`), that's not a single quote.
You might run into the following error when you first do so:
ERROR 1558 (HY000): Column count of mysql.proc is wrong. Expected 20, found 16. Created with MySQL 50077, now running 50148. Please use mysql_upgrade to fix this error.
To fix this, run the following on the server's command line.
# mysql_upgrade -h localhost -u root -p
For Plesk environments, you can use
# mysql_upgrade -h localhost -u admin -p`cat /etc/psa/.psa.shadow `
This will update the necessary database entries to the latest version of MySQL. Chances are, this will even have fixed the '#mysql50#' database prefix for you. If it hasn't, return to the first step here.
More information can be found at the bottom of MySQL's Documentation on the ALTER DATABASE command.