I'm sure there are a number of causes for this but this issue just came up on our support forum so I thought I would post the solution here for anyone else facing the same issue.
The solution itself actually comes from this post on the Joomla forum but it's a little tricky to find it, as it's mixed in with all of the Joomla admin password reset posts etc.
The user enters the correct login details and presses enter and the screen just refreshes again without any error or warning messages.
In the case that the username or password is wrong or the user does not have access to the administrator there is usually an error message warning the user of this. However in the case of this issue, there is no feedback to the user.
It turns out that this relates to the site cookies or at least an incorrectly set path to the cookie domain int he configuration.php file.
The solution is simple but it requires ftp access to the site.
1. Navigate to your site using your favourite ftp client.
2. Open the configuration.php file in your favourite text editor.
3. Reset the value assigned to the cookie path so that the code looks like this.
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