What makes the monitor not power up, and the power light turn yellow.

If its an LCD monitor, I had an issue once with the power cord in the back becoming loose. The power light would light up so there was some power, but the screen stayed black. What I found was that since LCD power cords usually plug in upwards in the back, they can work themselves out if the monitor or cord is moved around, or just from gravity I suppose. After I pushed the plug all the way up, my screen came back.