I have a notebook with Windows 10 64bits and he runs perfectly the Unreal Engine 4.10, but I installed the new version 4.12 to update my version of my game. So after installing the V4.12, and open the Unreal Editor, in the loading screen the editor stuck in 45% and doesn’t open the project.
I already uninstall and install the Unreal Engine 4.12 twice.
The first time you try to open a new version of the editor it will re-compile all of the shaders and stall at 45% for some time. Depending how large your project is (and how fast your CPU is), it can take anywhere from 2 to 30 minutes to complete.
To check and make sure the editor is still loading, right click on your taskbar and select Task Manager, you should see your CPU usage at or near 100%, and in the list below there should be a few ShaderCompilier threads. If so you just have to let it finish. =)
I have had this issue to.
Although I think this issue is already fixed, just a suggestion for in the future for other people experiencing this problem, it may be windows defender you got to disable.