After Upgrading My Site I Can No Longer Log In

Not being able to log in after a site upgrade can be a traumatic experience. Rest easy - it usually is simple to fix.
The first thing to check is that you copied over your MachineValidation and MachineDecription keys from your previous web.config. When upgrading one of the steps you need to perform is to move all your custom settings from your old web.config to the new version, and these keys can be easy to miss.
If you still can't login with the old password after that, try registering a new user on your site. After doing so you can query the dbo.aspnet_membership table to get the password and passwordsalt from the newly registered user. Next update your host user's values with these new values. Now you should be able to login with the new user's password.