You can't retrieve your password (even with PhpMyAdmin) because it's encoded in the DB.
The PhpMyAdmin credentials are usually the same as your cPanel Login. If not, you'll have to ask your host to reset your PhpMyAdmin password.
Once you can get into PhpMyAdmin, you can follow the tutorial, or if that doesn't work, you can create a new, separate installation of the exact same version of MODX (anywhere). Go into that new DB with PhpMyAdmin, select the modx_users table and look at the first record (that's you).
In another browser window, open the broken install's DB in PhpMyAdmin and carefully cut and paste all the fields, one-by-one from the first record in the modx_users table from the new install into the same record of your broken version of MODX.
Then, delete all the files in the core/cache directory (if you're using Revolution) and you should be back in business.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html
