iMac screen goes black after TDM

Hi

My iMac 2013 goes black when disconnected from my MBP in Target Display Mode. I have tried unplugging the cable and CMD+F2, No difference.

I can hear it run and share screen to my MBP from “share screen” in Finder on MBP. I can use the mouse and keyboard, but there is no way i can get the screen back on.

Both systems is 10.13.1.