Possible reasons are-
a) user has been locked.
b) There are too many users more than specified.
c) Your System/Ip (rights were provided to which) has been changed.
Please follow below steps-
Step1: Please server connect with root user and check if you are able to use your DB.
Step2: execute below commands.
mysql> flush hosts;
Now try to connect from phpmyadmin if success then fine other wise check if you are getting any IP related error if yes then check if that ip have required permissions.
Other Reasons may be
- opened_files_limit crossed whatever you mentioned in your configuration file.
- disk full.
- too many thread opened/too many connections.
- your login_user has been locked due to wrong password input a certain times (limit mentioned in configuration file default is 10).
- mysql port (default is 3306) is not open on DB server from outside.
- user does not have permissions from your system IP. etc.