I had exactly this same thing happen to my pi-top  just the other day. Actually, in the right light, I could tell that there was something on the display, but otherwise it just looked dark. I sent a note to the support email link, and got this very useful reply below back. About 30 minutes of careful work and I was back up and running.
"It sounds to me as though one of the cables from the hub to the screen is a little loose and the power for the backlight isn’t being sent to the screen. It may be a simple fix though it will take a little handiwork from yourself to test it out, would that be okay?
You’d have to remove the cooling bridge and slide out the hub from the Raspberry Pi as well as disconnect power. Then remove the 4 screws that connect the hub to it’s sliding rail., careful that the screwdriver does not make contact with the other components on the board. Once you’ve done that, carefully spin the hub, so that the hdmi connector comes up. Be careful of the cables that are still connected, don’t disconnect any of them yet. You’ll see two sets of cables, one at the bottom of the hub, and two towards the top, don’t disonnect the bottom cable (it’s marked with a warranty sticker).
Carefully disconnect both of the cables at the top of the hub bu pulling the black bar towards the cable andout of the connector. It’s a delicate connector and you don’t need much force. Do the same with both cables, but careful to remember which cable does into which connector. Then reinsert both cables back into their respective connectors as far as they will travel inside and push the black bar into the connectors to lock the cables in place and flip the hub back and secure it back in with screws and attach the cooling bridge.
Reconnect the power cord and check if the screen is now working."