Euroway Prusa I3X Not Starting

I just assembled a Euroway Prusa I3X. It has a dual 12V output PSU and a GT2560 board.
The first time I powered it on (after rechecking all the connections) the LCD flashed on then immediately went black.
Now when I power it up nothing happens at all. If I hook it to USB then the LCD lights up, and I can access all the menus. It still doesn't activate the steppers or the fans, but I can heat up the bed and extruder. I've checked the PSU, and I have 12V on both V+ legs. The power LED on the GT2560 board lights up. When I plug in the USB the other LED on the board lights up. I checked both of the tiny fuses and they're passing voltage. If I disconnect the USB then the LCD goes dark regardless of whether I have the power switched on.
Any clue what I'm missing?

Tony

Check the protection diode in series with the power input, it may be defective.
Remove all motors and stepsticks, endstops etc and connect the 12V psu. If that works keep adding stuff one by one and check if it's still working. If you hit on something that makes it stop working you can try replacing it.

Well, must have been the Xyrtec talking.
When I re-checked the fuses I found that they didn't have continuity. Swapped in new fuses, and everything lights up an runs now.
On to setting this thing up and figuring out how to make stuff.
Wheee!