What size of SD card are you using? It should be 8 or 16 GB and class 10. If you can, please try another card from a different manufacturer. Also maybe try a different power source.
To make certain it's not just a monitor / display setting issue please review this note from the FAQ:
My display doesn't look right, the screen appears cropped or stretched
This can happen on some monitors. To work around this force the board into DVI mode (vs. HDMI mode). You can read more about this here viewtopic.php?t=5851
. Mount the SD card to a local PC and proceed to edit the file in the EFIESP partition. For example if the partition is mark as H drive, edit H:\config.txt by adding the following line.
hdmi_group=2 # forces DVI timing to be used
Jonathan Tanner | Microsoft | Windows 10 IoT Core Insider Preview Support | This posting is provided 'as is' with no warranties and confers no rights.