Locked out of your own application.
FIRST CHECKS
Is the password correct? Reset it rather than assuming. Is the URL correct? Some applications use a non-obvious admin path. Are cookies enabled, and are you on the same hostname the application expects? A mismatch between www and non-www causes login loops.
RESETTING THE PASSWORD
Most applications provide a password reset by email, which requires mail to be working.
If mail is not working, reset the password directly in the database using phpMyAdmin, applying the correct hashing function for that application.
Many platforms provide a command-line tool for this, which is safer than editing the database.
LOGIN LOOP
You log in and are returned to the login page. Usually a cookie or session problem, frequently caused by the configured site URL differing from the URL you are visiting.
Set the site URL explicitly in the configuration file.
IF YOU ARE BLOCKED BY A SECURITY PLUGIN
Rename the plugin directory over SSH or File Manager to disable it, then log in and correct the setting.
IF NOTHING WORKS
Open a ticket. We can check whether the problem is server-side, though we cannot reset application passwords for you.