I replaced the LCD on a 2015 Retina MacBook and now the laptop isn't showing anything on the screen except a battery...

This laptop experienced water damage and I was able to clean it all up. The laptop works fine except there is no backlight...

If you're able to get display out fine then the issue lies with the LCD panel. Unfortunately you're going to have to replace it. As for the heat though, I would blow out as much dust as you can from the inside while you have it open with canned air, and make sure you get the back vent behind the stand as well.