Have you tried pressing Alt-tab to ensure the login prompt is hiding behind the game window?
Also, please make sure you type your login and password correctly (including capitalization). If you still encounter this issue, try moving your prefs.prop file from your save directory to a new location and then logging in again. If this works, then your prefs file may be corrupted. If this is the case, then it cannot be used, which will also prevent the use of your current save games.