In the case where a member/user cannot login to the website the first thing to do would be to use the 'forgotten password' function available on the login screen.
The user should enter their email address and, if their email address is found in the user records, they should then receive a password reminder email.
If the user still cannot login there are then a few things that a site admin can do to check and potentially resolve the situation. In order to do this they will need to find and open the user record in the admin area.
1: Check the users record to see if the member account does exist and isn't suspended
2: Check that the login details the user is using are correct (this will either be email address or membership number).
3: Reset the password, on behalf of the user, to something that is agreed with by the user.
If this still has not resolved the situation, please contact email@example.com and we will look into this for you.
In many cases the underlying cause of a user being unable to login is simply that the user has forgotten their details (even if they are very confident that they have it correct). In other cases it can be because there is a problem with the users details that can be rectified on a one-off basis (e.g. an email address was entered incorrectly).
However in some cases login problems are caused by a problem with a third party integration. If it looks like the problem could be related to an integration (most tellingly it will be likely affecting multiple user accounts) then please contact firstname.lastname@example.org and we will look into it as a priority.