Why does Screen goes black after it was taken off and put back on?

I just recently replace a proximity sensor cable. I was very careful with every part of the repair. now that everything is reassembled, I am having an issue. the device will power up and I can unlock it, touch works, everything is normal. I can lock the phone, then hit the home button or power button and I can feel the phone vibrate ( so I know its responding) but the screen stays black. I have to reboot it to get it to turn off and back on again. any ideas?