Windows 10 Technical Preview Build 10049 is available now. But it also has issues like the predecessor. One of the most common problems you will face is the a blue screen instead of the desktop when signing into Windows 10.

How to fix blue screen issue and get this desktop back
How to fix blue screen issue and get this desktop back
To work around this issue, lock your PC (with the hardware button, or by pressing the Windows Key + L) and try signing in again. 
You can also try pressing Ctrl+Shift+Esc to open the Task Manager, and then look for the lock screen (lockapp.windows.exe) listed in the processes with a “Suspended” status.  
Select the process and click End Task.
Via Microsoft Windows Answer