S2421HS, DP in port not working from HP laptop?

@pnjohns  Are you using basic DisplayPort cables, i.e. with a DisplayPort connector on both end rather than DisplayPort to HDMI cables, which cannot be used to connect a PC HDMI out source to a DisplayPort input?  And are you actually switching the input on the display over to DisplayPort when you want to use that input?  Sorry if these questions sound trite, but you'd be surprised how many people get tripped up by those two items.

@pnjohns  Ok, well that explains it.  Fyi when asking for technical assistance, it helps to provide basic technical detail, like the specifics of the cable types you're using, so that the people here who are willing to help have something work with.

In terms of what's going on here, I'm amazed at how often this comes up.  Typical DisplayPort to HDMI cables and adapters can only be used to connect a DisplayPort output on a PC/dock to an HDMI input on a display. They cannot be used to connect an HDMI output on a PC/dock to a DisplayPort input on a display.  This is because most -- but not all -- DisplayPort outputs support a capability called "Dual Mode DisplayPort" that allows them to fall back to native HDMI signaling to feed an HDMI input.  But HDMI outputs do not support sending a DisplayPort signal, and you cannot send a native HDMI signal into a DisplayPort input.  There are a very small handful of adapters that are specifically designed to go in the opposite direction of connecting an HDMI source to a DisplayPort input, like this one.  It incorporates an active signal converter chip to achieve this "opposite" conversion.  The USB connector is to provide power to run the converter circuitry.  But as you can see it's a bit different and more expensive from your typical DP to HDMI cables and adapters.  Note that if you choose not to go with that specific product, be aware that there are also "active" DP to HDMI cables/adapters designed to go in the more traditional direction of DP source to HDMI input.  Those exist in order to be used with DisplayPort sources that don't support Dual Mode DisplayPort and/or where that isn't allowed, such as AMD EyeFinity setups.  Those types of active adapters are simpler and less expensive than the one I linked, but they are NOT suitable for what you are trying to do.

Alternatively, if your laptop has a USB-C port that supports video output (laptop model information is also good basic technical info to provide upfront for next time, even if it's a non-Dell laptop), then you can use the much simpler option of a USB-C to DisplayPort cable.  But not all laptops with USB-C ports support video output through them.