This is actually an example question. I get a number of questions like this from time to time. The scenario is this: you have something like a laptop whose screen has failed. In other words, you can’t see what’s on your computer. One of the very common approaches to try and work around that until it gets fixed or replaced is to connect to an external monitor. The problem is that the external monitor doesn’t show anything.
There are a couple of additional steps that might be necessary, beyond just plugging in the monitor.