I recently encountered a problem in which it was impossible to login to the WordPress backend/admin area since the password field was not accepting any input. The same problem occurred on all the browsers I tested it in. It seems like an update might have broken something in a theme or a plugin. Finally there was an easy solution to the problem:
Just open the developer tools of your browser, target the password input field and delete the bit where it says “disabled” from the source code (displayed typically on the right side of you browser window in the Developer tools panel). Now the form field takes input again and you are able to login and update your plugins/theme if needed.