Hi there and welcome
With the Laptop and monitor connected, press the FN + F5 and see if they both display. If not, repeat 5 times and check each time and tell me if you can get both to display please.
If you right click on the desktop and choose Screen Resolution
How many monitors do you see there?
When did this problem start? How many days / weeks ago
Can you close all programs.
Click start, type in
follow the onscreen steps and perform a system restore choosing a restore point from before Monday.
Let this complete and when it has, tell me if the monitor is now working please.