One of the more common problems for users and administrators of Joomla 2.5.x based websites is the issue which prevents users to log in, no matter if they try front or back end. For example, you create new user in back end and assign it to correct group. Everything looks OK, but if you try to log in with the credentials you just used to create this new user, Joomla will not comply. Instead, you will see this error (in back end).
Username and password do not match or you do not have an account yet.
There are several reasons potentially causing this problem, from obvious mistakes in username and/or password typing to some not so obvious. One of the latter actually solved the problem for us.
SOLUTION
Log into back end of your Joomla 2.5.x site and look for "Authentication - Joomla" plugin in "authentication" group. If this plugin is disabled, enable it and you should be able to access Joomla front and back end with newly created user credentials.