I have the same problem on my laptop with my Windows 10 upgrade with Sandy Bridge hardware, HD 3000. On boot without a monitor attached, the internal screen is blank, but if I hit the power button to put it to sleep, then hit it again to wake it up, the internal screen comes on and I can see the login screen. This is a pain in the ****. Microsoft approved the upgrade to windows 10 on my hardware, so Intel needs to upgrade support for Sandy Bridge as well.
Try with the latest generic Intel® HD graphics 4000 driver version 188.8.131.52.4252 for Intel® HD graphics 4000
In regards to Intel® HD graphics 3000, Microsoft® Windows 10 is not supported yet.
Supported Operating Systems
This information is intended to all Windows 10 users that are experiencing issues with a supported Intel® Graphics Controller (supported graphics)
See post # 93