How did you have the T100 connected to the desktop? Do you mean that you had it plugged in via HDMI to the monitor, or with USB to the actual desktop? If it was with USB, than you won't be able to project it to the other screen. I am assuming though that you were using the HDMI output on the T100 and had it connected to the Desktop Monitor.
First make sure that the cable is plugged in all the way, and that the monitor is on. When you select to project the the screen on the T100 to the desktop monitor, you may have to toggle the power button on the monitor it was in sleep mode. In order to get the display to return to the T100's screen hold down the Function (fn) key, and press F8 one time. This should set your T100 back to the default screen setting and it should show up on the tablet screen again. If you still can not get it to change back, then unplug the HDMI cable, and then hold down the Home Button (the one on the side under the volume button) and the Power Button for about 10 seconds. This should force your T100 to reboot and reset the display settings to default. Let me know if you need help with any of this, or if these steps didn't work for you.