btbuses This could be related to a few things. Logins rely browser cookies, so if something was wrong with your browser cookies at the time (locked, insufficient storage, broken browser plugins), that could then have ben resolved by restarting the browser, or even restarting the system.
Alternatively, since logins rely on the server SESSIONS (PHP), it could be there is a problem with sessions on the server (insufficient space, misconfigured sessions, non-persistent sessions). If the issue was resolved when rebooting however, it's more likely it was a browser-related issue.