Hi,
I've been using MySource Matrix without any problems from the _admin until this morning, when I tried to key in my usual admin pass, I get an invalid username or password as well. What do you suggest I look at?
Thanks,
Ken
DO you use a local account or an external account like LDAP? Is this the root account? If not, do you know the root password?
Firstly please verify that you haven't inadvertently hit Caps Lock when entering either of these fields. It might also be worthwhile trying this in another browser.
Otherwise I would suggest contacting a user with higher access (eg; a System Administrator if you are a Backend User, or the Root User - if you are a client the latter may be us here at Squiz) to log in and reset your password. Another possibility is that your user account is Under Construction which can also be verified by one of these users.
If your site contains a Password Reset page, you might be able to log in again by providing the email address assigned to your account. An email will be sent which will contain a link to allow you to change your password.
Edit: After following Greg's instructions above 
Just an idea, sometime systems block users after multiple login attempts. So if you think someone maybe tried to login with your password - you might want to log in using ROOT and see if that's the case…