Probably the next step is unfortunately to take the unit apart again.
Verify that the microSD card is properly seated, and didn't come open during assembly.
Check the board for any signs of mechanical dysfunction, i.e. lifted connectors etc. that can happen during assembly.
When you plug the device in, at the very least one of the lights to the clear bezel should turn on, as it's turned on by the very low level bootloader.
If you have any experience with linux and/or have a serial cable that does 3.3V TTL levels you can do further debugging.