I have manually reset a user's password, but the user still cannot login
Check (in Admin -> Global Settings -> Global Permissions) that the user belongs to a group that has the JIRA Users permission.
The user cannot get past the login page. After clicking the "Log In" button, the login page just refreshes.
This usually occurs when JIRA cannot set a browser cookie. Ensure that cookies are allowed in the user's browser settings.
If you are using IE6, check that your server name does not have an underscore ("_") in it, as IE6 has a problem with this (see JRA-1624).