Hi Anthony,
As @timtomissus suggested, you'll want to open the phone back up and double check your work. Sometimes those long FPC connectors feel like they're connected but still need a bit of fiddling to get them 100% seated. Check the motherboard connector and the cable connector for any bent or broken pins, then clean both sides with some 90% or higher isopropyl alcohol.
If it still doesn't work after that, you might try verifying whether the old screen still works or not. If it does, you may have a defective replacement; it happens on occasion.