U2415, no DP cable message

I just purchased a U2415 monitor. I hooked it up to my computer via an HDMI cable. Now the computer says "No DP Cable" and goes to sleep and won't wake up. I've tried different HDMI cables but it still goes to sleep without waking up.  I tried both HDMI ports, but no change. I tried an old Dell monitor and it worked fine. Still, I'd like to use my new monitor. Any ideas?  Thanks!