changing the machine key should cause you to be logged out because the authentication cookie is generated using the machine key.

you should be able to login again after changing the machinekey as long as passwords are not encrypted. If you cannot login, I suggest close all open browser windows except maybe google (definitely do not have your site in an open browser window), clear your web browser cookies (it won't clear cookies if the site is still open in a browser window that is why you must close it). After clearing your cookies go back to your site and you should be able to login. If not then start with basic troubleshooting to look for any errors.

also make sure you don't have extra copies of web.config file lying around in other folders, that can interfere with the main web.config and cause problems especially if there is a machinekey in the other web.config file