|What happens whin you hit the power switch? Any lights, sounds, Describe?|
What were you doing when, or just before, this happened?
Describe your Computer OS, Processor, RAM installed,
Opened the Case for any reason?
Moved the Computer?
Checked the Monitor connections?
Does Monitor have power?
Does the Computer have power?
Lights on Front of Monitor?
Lights on front oF Computer?
Try another Monitor?
What accessories are connected? USB, Printer other?
If you can answer most of these someone should be able to assist you
As a Start, disconnect everything except the Monitor, Keyboard and Mouse, Check all external conections and try again, noting what you see and hear after you power up.
There is nothing to learn from someone who already agrees with you.