Monitor problem

[adgjlsfhk]

I recently built a new computer (late April) and everything worked great. I am using an old 19" Dell monitor which I am going to upgrade soon. I tried to turn the monitor on, but the light on the power button for the monitor just flashes on and off. I can not turn off the monitor except by unplugging it from the power source. The power chord is not the problem because I still have the chord from my old computer and it does not work with it. Any advice?

 

[adgjlsfhk]

I fiddled around with some screws and it now works, but can anyone tell me what was wrong, and if it will happen again?