iMac shuts down after removing Mini DisplayPort to SVGA

Hi,

The last couple of years I have worked with a 27 inch iMac in combination with a HP 27 inch monitor. This monitor is connected via a Mini Displayport to SVGA connector. Because I am going to use this external monitor anymore, I disconnected it today which made my iMac shut down imediately. Then it keeps rebooting and trying to start up but shuts down again. Over and over.

The problem stops as soon as I connect the monitor again.

Anybody have any thoughts?