Sitemap, URL prefix change

To attempting to get my sitemap to work. I was just curious how safe it is to change the wordpress URL in the General Setting?…I changed the address by adding www to the domain. When I saved the change wordpress went to the log in page and wouldn’t accept my password. I used the back button and it got me to the Settings page so I changed it back and saved it again. Now it accepted my password. What’s going on? And on that note,,is it possible to ever get back into the dashboard if you were to forget your password?

Thanks,
Mike