Macbook air, 11 inches backlight not working after lcd replacement
You'll have a much better vantage point than us on the internet to guess.

First, reconnect all cables and hope it's that.

If not, you'll need to start replacing parts one by one. The odds of a new LCD being faulty are low - but the circumstances of the repair might help.

Was the LCD out the reason you swapped the panel?

You might need a second machine to isolate each part - test whether it's the cable, inverter, logic board LCD or something else.