Hi, NigelPop
Welcome to Microsoft Community.
We understand that you have a problem that you can't get feedback from the monitor when you use your laptop. According to your description, it seems that this is caused by the interruption of the monitor signal connection because the computer has not been operated for a period of time after you go out. Before that, please check whether your monitor cable and external monitor can work normally. This may also be caused by hardware damage.
Next, please try to keep pressing Windows+P until you can see your system desktop on your monitor (Windows+P is a shortcut key to switch your display mode).
I hope this will help you!
Sincerely
Nicholas | Microsoft Community Support Specialist