If there is no signal or no connection between the PC and the VGA jack/HDMI socket of the TV set when you switched from TV to PC mode, the TV set will show a blank screen after a few minutes.
To avoid this, check that the VGA Video / DVI cable from the PC is conected to the VGA jack / HDMI socket on the bottom of the TV or ensure that you have changed the video mode on your PC when you switched over to PC mode.