Consider the following scenarios:

• An additional monitor is connected to a Windows Vista-based desktop computer. The additional monitor is designated as the primary display (the main monitor). The computer goes into hibernation or goes to sleep. This additional monitor is disconnected. The computer resumes from sleep or from hibernation.
• An external monitor is connected to a portable Windows Vista-based computer. The additional monitor is designated as the primary display (the main monitor). The built-in screen is designated as the secondary display. The computer goes into hibernation or goes to sleep. This external monitor is disconnected. The computer resumes from sleep or from hibernation.

In these scenarios, you cannot see the logon screen. Therefore, you cannot log on to the computer.