An undetected bug hit Windows 10 Technical Preview Build 10049 shortly after it was released to Fast Ring'ers on Monday. Shortly after it was released and installed by Windows Insiders, some found that Hyper-V stopped working.
Microsoft has confirmed that this bug does exist under a certain condition, and has supplied a workaround.
The problem happens if you didn't have Hyper-V enabled prior to installing Build 10049, but then tried to enable it after the upgrade. If you were already using Hyper-V with the previous Build, all is well, and Hyper-V works as expected.
The workaround involves resorting to installing an earlier Build of Windows 10, enabling Hyper-V, and then upgrading again to Build 10049.