MacBook screen blank

Sounds like you must have missed the connector from LB to screen...or it worked loose when you put the Mac back together....There's only one way to check...Open it up again.

 

There might be some dust in between the connector, clean it with a small brush.

If you have a monitor I would also try to connect it to the monitor, if you get output there your Logic Board is OK.

 

Maybe a crimped or damaged cable comes to mind...sometimes it's impossible to tell. As long as you were grounded when doing the repair, I can't see how you could have fried the GPU, but that's what comes to mind next.

 

I think that it's two possibles now....A bad GPU, or ( hopes ) a bad cable...Over time, those cables can become brittle inside...you'd never know by looking, but a replacement shouldn't cost too much....hope it's not your GPU.

 

Yes, if the internal GPU will display correctly on another monitor the odds are that you just damaged the cable....A replacement should do the trick...Let us know how you go...