Hi,
This symptom is indicative of a "Faulty monitor". As a basic test, do you have access to another monitor that you could connect & see if you are able to work as normal. If so, then the fault lies with your defective monitor.
Please note: -
If you swap video cables & the monitor, then the fault could be in the cable & the monitor is still good!
Let me know how you get on & if there is any other help needed post back & I will assist further.