Computer crashed after delete blueprint class drived from C++

I am learning UE4 programming follow the tutorial to here link text, But UE4 crashed while I try to edit a floor.
Fatal error: [File:D:\Build++UE4+Release-4.15+Compile\Sync\Engine\Source\Runtime\Windows\D3D11RHI\Private\D3D11Util.cpp] [Line: 181] Unreal Engine is exiting due to D3D device being lost. (Error: 0x887A0007 - ‘RESET’)

To find the bug,I deleted blueprints drived from C++,but UE4 crashed more frequently.at last computer crashed.