My second monitor isn't being detected by my computer

I have a very old monitor that I wanted to use as a second monitor to read stuff on while I play games on my main monitor but the problem is that the older monitor doesn’t have HDMI. So I had to use a DisplayPort which is fine but my computer isn’t detecting it. I don’t know why but even when I try to use the old monitor stand alone it doesn’t work although I used to use this same monitor with a DisplayPort and it worked but now it just isn’t working (And it’s not that the cable is broken or that the monitor doesn’t work, it’s telling me that the monitor is connected, but no signal from the computer). I think it’s because my graphics card isn’t detecting it but I don’t know if that’s actually the reason, and if it is, I don’t know how to fix it.

Sorry if this was worded poorly, but can anyone help me?
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