Second Display Help

Hello.

I recently purchased a second display (HP 27VX) for my iMac. I have a 27" Late 2009 running El Capitan 10.11.

I'm using a mini DisplayPort to HDMI and can't get the second display to show up. It recognizes it in System Preferences, and gives me options for resolutions but the monitor itself stays black and when I cut it off and back on, it only shows "monitor going to sleep" before going black again.

Any help with this would be amazing. thanks!