Such problem occurs when password for Plesk admin  within MySQL does not match with saved password within system.

You can fix it yourself using the following instructions.

First
of all you must know you current Plesk password. If you have
forgotten it by some reason then password can be taken using %plesk_bin%\plesksrvclient.exe -get

Then you should set it up within Plesk MySQL. To do it please follow these steps:

1. Edit MySQL configuration file my.ini located within %plesk_dir%\mysql\data\.
2. Add parameter skip_grant_tables=1 into [PleskSQLServer] section.
3. Restart service Plesk SQL Server.
4. Log into Plesk MySQL.

   
   %plesk_dir%\mysql\bin\mysql.exe -P8306 mysql

5. Execute the following query:

   mysql> update mysql.user set password=password(‘current_password’) where user=’admin’;

6. Remove skip_grant_tables=1 from within [PleskSQLServer] section within %plesk_dir%\mysql\data\my.ini
7. Restart service Plesk SQL Server.

New password has to be applied.

Note: my.ini for MySQL on Plesk 7.0 is located within %SystemRoot% (usually it points to c:\windows\)