Because we are not able to see or access your hardware, and because of the vast range of abilities of people who come to Computer Hope for help, we cannot make any assumptions about your situation. Hence all the questions we tend to ask.
The first thing that you were asked (although you could easily have overlooked this) is to provide more information about your system. (GX1_Man said "see below" - which meant, refer to his signature asking for more information.) What monitor? What type of connection? What graphics card? Does the monitor work fine with another PC? (I think you've said "yes" to this one.) Do any other monitors work fine with the computer?
The system supplied to you new should work, as supplied. If the monitor and PC don't work together, then your first port of call is the people who supplied you this "new" hardware. That is exactly what GX1_Man was saying. So yes, I do think his "answers" are right.