Update your post to include full system hardware specs and OS information.
Motherboard, CPU, GPU, etc....
Include PSU: make, model, wattage, age, condition?
New RAM: did you swap out old RAM or add RAM? RAM specs and sources?
Disk drives: make, model, capacity, how full?
Make and model monitor? What connectivity (e.g., HDMI, DP, DVI) is being used?
Have you tried another known working video cable?
Does another known working monitor work on your computer?
Does the monitor work on another known working computer?
Any pop up windows with error messages or other video related errors being presented?